International Journal of Heat and Mass Transfer

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1 International Journal o Heat and Mass Transer 55 (0) Contents lists available at SciVerse ScienceDirect International Journal o Heat and Mass Transer journal homepage: Photographic study and modeling o critical heat lux in horizontal low boiling with inlet vapor void Chirag R. Kharangate a, Issam Mudawar a,, Mohammad M. Hasan b a Boiling and Two-phase Flow aboratory, School o Mechanical Engineering, Purdue University, 585 Purdue Mall, West aayette, IN , USA b NASA Glenn Research Center, 000 Brookpark Road, Cleveland, OH 4435, USA article ino abstract Article history: Received 4 June 0 Received in revised orm 8 February 0 Accepted 8 February 0 Available online 0 April 0 Keywords: Flow boiling Two-phase pressure drop Critical heat lux (CHF) This study explores the mechanism o low boiling critical heat lux (CHF) in a.5 mm 5 mm horizontal channel that is heated along its bottom.5 mm wall. Using FC-7 as working luid, experiments were perormed with mass velocities ranging rom kg/m s. A key objective o this study is to assess the inluence o inlet vapor void on CHF. This inluence is examined with the aid o high-speed video motion analysis o interacial eatures at heat luxes up to CHF as well as during the CHF transient. The low is observed to enter the heated portion o the channel separated into two layers, with vapor residing above liquid. Just prior to CHF, a third vapor layer begins to develop at the leading edge o the heated wall beneath the liquid layer. Because o buoyancy eects and mixing between the three layers, the low is less discernible in the downstream region o the heated wall, especially at high mass velocities. The observed behavior is used to construct a new separated three-layer model that acilitates the prediction o individual layer velocities and thicknesses. Combining the predictions o the new three-layer model with the interacial lit-o CHF model provides good CHF predictions or all mass velocities, evidenced by a MAE o.63%. Ó 0 Elsevier td. All rights reserved.. Introduction Flow boiling is widely used in applications demanding the removal o high heat luxes while maintaining low surace temperatures. These include nuclear reactor cores, lasers, advanced microprocessors and hybrid vehicle power electronics []. A primary design goal in these applications is to ensure coolant operating conditions that yield critical heat lux (CHF) values saely above the dissipated heat lux. For applications where the heat dissipation is heat-lux controlled, exceeding CHF can potentially lead to catastrophic ailure o the device or system because o the ensuing uncontrolled rise in surace temperature. When implemented in most o the these applications, the low is maintained mostly in subcooled state to capitalize upon the coolant s ability to cool suraces by sensible heat absorption in addition to its latent heat o vaporization. However, maintaining subcooled conditions is not always possible in a two-phase cooling system. For example, a single two-phase loop may be used to cool an array o heat dissipating modules in series, in which case the incoming subcooled liquid gradually loses sensible heat as it lows through the system. So, even i upstream modules can capitalize Corresponding author. Tel.: ; ax: address: mudawar@ecn.purdue.edu (I. Mudawar). UR: (I. Mudawar). upon the high sensible heat content o the coolant, downstream modules may contend with only saturated low boiling. For the latter, the coolant may enter the modules with a inite vapor void raction. It is thereore very important or this cooling scenario to be able to predict CHF or saturated low boiling with appreciable vapor content. Exploring such operating conditions or horizontal low is the primary goal o the present study... Flow boiling critical heat lux Numerous studies have been published over the years that address horizontal low boiling and CHF. In general, upward acing heated walls are preerred or horizontal low to take advantage o the buoyancy orces in helping remove vapor rom the heated wall and replenish the wall with bulk liquid, two mechanisms that are vital or sustaining nucleate boiling and delay the ormation o the vapor blanket known to precede CHF. These beneits are especially important or low velocity lows, where weak liquid drag magniies the inluence o buoyancy in a horizontal channel. Identiying the trigger mechanism or CHF in low boiling has been the subject o intense research or many decades. Four dierent categories o models have been suggested, boundary layer separation, bubble crowding, sublayer dryout, and interacial lit-o. The boundary layer separation model is based on the assumption that the near-wall liquid velocity gradient becomes vanishingly small /$ - see ront matter Ó 0 Elsevier td. All rights reserved.

2 C.R. Kharangate et al. / International Journal o Heat and Mass Transer 55 (0) Nomenclature A cross-sectional o low channel b ratio o wetting ront length to wavelength C 0 void distribution parameter C,i interacial riction coeicient C j empirical constant in riction actor correlations, j = 3 CHF critical heat lux D diameter D h,e equivalent heated diameter riction actor G mass velocity g gravitational acceleration H height o low channel h enthalpy h g latent heat o vaporization HEM Homogeneous equilibrium model Dh sub,i inlet subcooling heated length MAE mean absolute error P pressure P h heated perimeter P i interacial perimeter P R reduced pressure in Bowring correlation P w perimeter in contact with channel walls q 00 wall heat lux q 00 m critical heat lux q 00 m0 critical heat lux based on channel heated area or zero inlet subcooling Re Reynolds number T temperature U mean axial velocity u i interacial velocity v speciic volume v g speciic volume dierence between vapor and liquid w width o low channel W total mass low rate We Weber number W 0 g x z z 0 z rate o liquid evaporation along lower liquid vapor interace low quality axial (stream-wise) distance axial location where vapor layer velocity just exceeds liquid layer velocity axial location or determining vapor layer thickness and critical wavelength in interacial lit-o model Greek symbols a void raction d vapor layer thickness k wavelength k c critical wavelength l dynamic viscosity q density q 00 modiied density r surace tension i interacial shear stress w wall shear stress Subscripts top vapor layer bottom vapor layer exp experimental (measured) saturated liquid g saturated vapor i inlet to heated portion o low channel; interace k phase k, k = g or o exit rom heated portion o low channel pred predicted tp two phase w wall when the rate o vapor production normal to the wall reaches a critical magnitude, which causes the liquid to separate rom the wall [,3]. The bubble crowding model is based on the postulate that CHF occurs when turbulent luctuations in the core liquid low become too weak to transport liquid through a thick bubbly layer [4,5]. The sublayer dryout model is based on the assumption that CHF commences once the heat supplied at the wall exceeds the enthalpy o liquid replenishing a thin sublayer that orms beneath oblong, coalescent vapor bubbles at the wall [6]. Proposed by Galloway and Mudawar [7,8] in the early 990s, the interacial lit-o model is based on the observation that, prior to CHF, the vapor coalesces into a wavy vapor layer that makes contact with the wall only in discrete wetting ronts corresponding to the vapor layer troughs, and CHF is initiated by lit-o o the wetting ronts rom the wall due to intense vapor momentum. More recently, Zhang et al. [9] explored the eects o low orientation relative to gravity on both subcooled and saturated low boiling in a rectangular channel that was heated along one side. For saturated boiling with zero inlet vapor void, their extensive low visualization experiments revealed several possible mechanisms or CHF. Occurrence o a given mechanism was dictated by the combined eects o low velocity, low orientation, and placement o the heated wall relative to gravity. Weak liquid inertia at low velocities permitted buoyancy orces to play a major role in inluencing both bubble coalescence and vapor motion relative to the liquid, resulting in a number o complex CHF mechanisms dominated by pool boiling, stratiication and looding. Furthermore, CHF values or certain orientations were much smaller that those or vertical uplow. For low velocity lows, CHF ollowed the Interacial it-o mechanism only or vertical and near-vertical uplow. Interestingly, once the low velocity was increased beyond a threshold value, strong liquid inertia dwared all buoyancy eects, and the Interacial it-o mechanism was observed or all orientations; CHF values were also virtually identical regardless o orientation. The validity o the Interacial it-o mechanism was conirmed in a subsequent study by Zhang et al. [0] that was conducted in parabolic light to simulate zero gravity. In the absence o buoyancy, the validity o the same CHF mechanism was extended to even low low velocities. Modeling subcooled low boiling is complicated by the partitioning o heat extraction rom the wall between the coolant s sensible and latent heat. Zhang et al. [] conducted a comprehensive review and analysis o prior subcooled low boiling data and identiied two earlier correlations by Hall and Mudawar [ 4] as most suitable or both microgravity and horizontal low in Earth gravity. Zhang et al. [5] used high-speed video imaging techniques and experimental data to extend the Interacial it-o Model to subcooled low. The original model was modiied by accounting or the partitioning between sensible and latent heat using a heat utility ratio term. Overall, modeling o low boiling CHF or saturated inlet conditions with a inite inlet void remains quite elusive.

3 456 C.R. Kharangate et al. / International Journal o Heat and Mass Transer 55 (0) Two-phase pressure drop Pressure drop or low boiling with saturated inlet conditions and inite vapor void is typically predicted using the Homogenous Equilibrium Model (HEM) or any number o Separated Flow Models (SFMs) [6,7]. HEM is based on the assumption that velocities o liquid and vapor phases are both equal and uniorm across the low channel s cross section, while SFMs permit dierences between the phase velocities. Most recent SFMs o the two-phase rictional pressure drop are based on the ormulation by ockhart and Martinelli [8] originally developed or isothermal two-component mixtures. Martinelli and Nelson [9] developed a method or determining the accelerational component o two-phase pressure drop. To predict the two-phase rictional pressure drop, Thom [0] ormulated two-phase riction multiplier relations or steam-water. Baroczy [] ormulated an empirical two-phase riction multiplier in terms o mass velocity, quality and physical properties or liquid metals and rerigerants. Chisholm [] modiied the original rictional pressure drop model o ockhart and Martinelli with an interacial shear term. Friedel [3] later provided updated correlations or two-phase riction multipliers using a large data base he amassed rom dierent sources. An alternative approach to predicting two-phase pressure drop or separated lows is to use the control volume method, where conservation laws are applied to control volumes encompassing the liquid and vapor phases separately, and later combined or the two-phase mixture. The control volume method proved highly accurate in predicting two-phase pressure drop or vertical separated low along short [8] and long heated walls [9,0,4 7]..3. Study objectives The present study will explore pressure drop and CHF associated with low boiling in a rectangular horizontal channel with an upward-acing heated wall. The ocus o this study is on saturated low at the inlet with a inite void raction, conditions that prevail in downstream heat-dissipating modules when a number o modules are cooled in series using the same low loop. Highspeed video imaging techniques are used to explore vapor coalescence, especially near CHF. Pressure drop data will be compared to predictions o previous models as well as a new control-volumebased model. CHF data will be compared against the predictions o popular saturated low boiling correlations and the Interacial it-o Model.. Experimental methods.. Flow boiling test module The low-boiling module used in this study consists o two plates o polycarbonate (exan) pressed together between two outer aluminum plates using a series o bolts. As shown in Fig., a.5 mm 5 mm rectangular low channel is milled into the underside o the top plastic plate. The heated wall consists o a 0.56-mmthick, 0.6-mm-long copper plate that is heated by a series o thick-ilm resistors. The heated wall is attached into a rectangular groove in the low channel s bottom plastic plate. To prevent leaks, a lexible Telon cord is inserted into a shallow o-ring groove in the upper surace o the bottom plastic plate. A honeycomb insert is placed at the channel inlet to straighten the low and break up any large eddies. A channel entry length 06 times the hydraulic diameter ensures that liquid low becomes ully developed upstream o the heated wall o the channel. Fig. shows the heated wall assembly consisting o six thickilm resistors that are soldered to the underside o the copper plate. Each resistor is 6. mm long by 4.0 mm wide and has a resistance o 88 X. Five thermocouples are inserted into shallow holes in the copper plate along the centerline between the resistors. Uniorm heat lux along the heated wall is ensured by both equal electrical resistance o the six resistors and by connecting the six resistors in parallel and using the same variable voltage power source. The thickness o the copper heated wall is chosen careully to achieve two requirements simultaneously; details concerning these requirements are provided in [0]. Since very thin walls can induce premature CHF, the copper plate thickness is set greater than the asymptotic thickness at which CHF becomes insensitive to the wall thickness. On the other hand, ast thermal response is achieved by avoiding a very large wall thickness. Using a copper thickness o 0.56 mm allows the wall to reach steady-state temperature between power increments in less than 5 s. These two requirements are based on the need to use the present low boiling test module in parabolic light experiments, where steady-state thermal response must be achieved within the 3 s microgravity duration o a single parabola. While perorming parabolic light experiments is beyond the scope o the present study, meeting those two requirements ensures accurate CHF measurement while allowing a complete boiling curve to be measured in a relatively short period o time... Fluid conditioning loop The desired operating conditions at the inlet to the low boiling module are achieved with the aid o a compact two-phase low.5 Transparent Polycarbonate Plastic (exan) Plates 88 Ohm Resistive ayer (covered with Glass Passivation) Al O 3 Substrate Solder Pads Solder ayer (96% Tin - 4% Gold Metallization) Thermocouple hole Oxygen-Free Copper Plate Thick-Film Resistor All dimensions in mm All dimensions in mm 0.56 Oxygen-Free Copper plate Fig.. Flow channel assembly. Construction o heated wall.

4 C.R. Kharangate et al. / International Journal o Heat and Mass Transer 55 (0) loop depicted schematically in Fig.. FC-7 is the working luid used throughout the study. A detachable accessory to the twophase loop is used to deaerate the FC-7 beore perorming a series o tests. In the main loop, the FC-7 is circulated with the aid o a variable speed pump. Exiting the pump, the low is passed through a regulating valve ollowed by a ilter, turbine low meter, and two in-line electrical heaters, beore entering the low boiling module. The irst inline heater is used to ine-tune the liquid temperature but maintain single-phase liquid low, while the second in-line heater is intended to achieve saturated low with a inite void raction at the inlet to the low boiling module. With this arrangement, the low quality at the inlet to the low boiling module could be determined rom measurements o power input to the second inline heater as well as temperature and pressure measurements both upstream and downstream o the same heater. Exiting the low boiling module, the two-phase mixture is routed to a condenser to convert the two-phase mixture to liquid state. A nitrogen-pressurized accumulator between the condenser and pump both compensates or any expansion or contraction o the working luid throughout the loop as well as provides a stable reerence pressure point or the loop s operation. Metal bellows inside the accumulator help manage any FC-7 volume changes in the loop..3. Instrumentation and measurement accuracy An array o instruments is used to measure power and luid conditions throughout the low loop. The temperature o the heated wall o the low boiling module is measured by a series o ive equidistant type-k thermocouples that are inserted into the copper wall as described earlier. Two additional thermocouples are placed just upstream and downstream o the heated wall. Pressure transducers are also connected to taps in the low boiling module 3 mm upstream and downstream o the heated wall. The wall heat lux is determined by dividing the total electrical power input to the thick ilm resistors by the wetted area o the copper wall. The power input is measured by a wattmeter. Another wattmeter is used to measure power input to the second in-line heater. A thermocouple and a pressure transducer are used to measure temperature and pressure, respectively, at the inlet to the second inline heater. The low rate is measured by a turbine low meter. Accuracies o the low rate, pressure, and heat lux measurements are estimated at.3%, 0.0%, and 0.%, respectively. The luid and wall temperatures are measured with thermocouples having an uncertainty o 0.3 C. The entire apparatus, including the low loop components, power and instrumentation cabinets and data acquisition system, is mounted onto a rigid extruded aluminum rame as shown in Fig...4. Flow visualization and void raction determination A Photron Fastcam Ultima APX camera system with a shutter speed o /0,000 is used or visualization o the boiling low along the heated portion o the low channel. A Nikon Micro-Nikkor 05 mm /8D autoocus lens provides the high magniication required to capture interacial eatures o interest. The camera is positioned in a side-view orientation normal to the side o the low channel. The low is backlit with a light source, with a semi-opaque sheet placed in between to soten and diuse the incoming light. As illustrated in Fig. 3, the low visualization study targets three regions o the heated portion o the low channel, inlet, middle and exit, each spanning 0 mm. Together, the three regions cover nearly 60% o the heated length. While images corresponding to the individual regions are sometimes smaller than 0 mm wide, all images presented in this paper or the inlet region include the upstream edge o the heated length, those or the middle region are centered at the middle o the heated length, and the exit region images include the downstream edge o the heated length. Determining the void raction is possible only when the vapor and liquid phases are clearly separated and also span the entire width o the low channel. This is possible only or mass velocities o G = kg/m s. A high mass velocities, a combination o interacial waviness, phase mixing, bubble entrainment in the liquid phase, and droplet entrainment in the vapor phase precludes accurate determination o the void raction. When operating conditions are avorable, a systematic procedure is adopted or void raction determination. Strong contrast between the separated liquid and vapor is used advantageously or this purpose. The captured images consist o 56 levels o gray scale. Image correction and adjustment o brightness and contrast shows the vapor and liquid phases clearly separated rom one another. To calculate the void raction, this technique is applied to a control volume o length Dz = 3 mm within the captured region spanning the entire height o the low channel above the heated wall. Image analysis sotware is used to calculate the thickness o the vapor layer in pixels. The void raction is calculated as the number o vapor pixels divided by the number o pixels corresponding to the entire height o the low channel. This calculation procedure is applied to many Data Acquisition System Flow Boiling Module Display Panel Power Control Two-Phase Flow oop Fig.. Schematic o two-phase loop. Photo o test acility.

5 458 C.R. Kharangate et al. / International Journal o Heat and Mass Transer 55 (0) Fig. 3. Inlet, middle and exit regions o heated portion o low channel used or video capture, and sequential images o inlet and middle regions or G = 335 kg/m s, x i = , a i = 0.54: images 0 and (c) images 0. thousands o images in a video ile. Typically, images are recorded at rames per second, and the camera recording time is limited to.04 s. Thereore, this procedure is repeated or 4 5 sets o images taken at dierent times. Void raction estimates or the dierent times are typically within only ±% o the mean value. Other than optical limitations due to the camera, error associated with the void raction determination comes in two main orms. First, the method used assumes that the low is two-dimensional, that is, the vapor ormation extends uninterrupted along the camera s viewing direction. This error is minimized by the short width (.5 mm) o the rectangular low channel. Second, this method could not account or minute bubbles that are entrained in the liquid layer or liquid droplets entrained in the vapor layer. 3. Flow visualization results Fig. 3 and (c) show video images o the inlet and middle regions o the heated portion o the channel just prior to CHF, designated as CHF-in the present study. Fig. 3 shows or each region a sequence o ten consecutive video images or each region; Fig. 3(c) shows the ten consecutive images that ollow. The time elapsed between consecutive rames is.04/04 s. Because o the lack o phase separation in the downstream region, images or this region could not be used or void raction determination and are not shown in Fig. 3. Both Fig. 3 and (c) depict a unique threelayer ormation in the inlet region. Images o the middle region show signs o mixing o the phases that leads to the aorementioned highly mixed low in the downstream region. The twophase low arrives at the leading edge o the heated wall separated into two layers, with the vapor residing above the liquid due to buoyancy. A third vapor layer begins to evolve immediately at the leading edge o the heated wall. There is evidence o liquid at the wall beneath the lower vapor layer, providing localized cooling to the wall. Image sequences captured at dierent times show temporal variations in the vapor layer s shape and thickness due to waviness in the two interaces between the three layers. In the inlet region, there is axial thinning o the lower separated vapor layer, bringing the middle separated liquid layer closer to the wall. Fig. 3 and (c) show the upstream lower separated vapor layer becomes even thinner and is conined to the wall, perhaps mixing with the liquid layer. The vapor layer thinning is

6 C.R. Kharangate et al. / International Journal o Heat and Mass Transer 55 (0) attributed to the axial increase in mean low velocity causing an increase in shear between the three layers. There are signs o mixing between the lower vapor layer and middle liquid layer because o buoyancy eects. This eect is more pronounced in the downstream region. Fig. 4 depicts low development or the inlet region with increasing heat lux up to 95% CHF. Initially, with no heat added, the low consists o two clearly separated layers, with the vapor residing above the liquid due to buoyancy. At 50% CHF, bubbles can be seen orming at the wall then being entrained into the liquid layer or sliding along the wall; the low consists essentially o a clearly separated vapor layer above a bubbly low liquid layer. The three-layer low becomes well developed at 95% CHF as bubble coalescence and axial growth o oblong bubbles culminates in the ormation o a airly continuous wavy vapor layer at the wall. Fig. 4 shows, or two mass velocities, the low behavior in the inlet, middle and exit regions at 95% CHF. Notice how, or each mass velocity, the three layers are clearly separated in the inlet region, while, or the middle region, the lower vapor layer is quite thin and appears to mix into the middle liquid layer, resulting in a seemingly two-layer low. This behavior continues or the exit region, albeit with a much thinner mixed wall layer, caused by the increased mean low velocity. As indicated earlier, buoyancy tends to mix the two lower layers as the low reaches the middle region. Fig. 4 shows the lower mass velocity accentuates the buoyancy eects. Fig. 5 and shows images o the boiling low in the exit region at 95% CHF (CHF-) and during the CHF transient (while wall temperatures begin to increase unsteadily) or G = 335 and 650 kg/m s, respectively. Using the same low boiling module used in the present study, Zhang et al. [9] previously showed that, with zero vapor void at the inlet, a wavy vapor layer orms along the heated wall at CHF-. The wall could still be cooled by liquid rom the bulk region through wetting ronts, troughs where the vapor layer interace makes contact with the wall. CHF ultimately occurs when intense boiling in the wettings ronts causes the vapor layer interace to separate ully rom the wall, preventing any urther liquid access to the wall. Fig. 5 and show how the strong shear orces in the present study, which are the result o the relatively large vapor void, greatly decrease the thickness o the wall vapor layer, making any identiication o near-wall eects quite elusive. CHF Transient CHF- CHF- CHF Transient 4. Experimental results G = 335 kg/m s x i = , i = 0.54 G = 650 kg/m s x i = , i = 0.64 Fig. 5. Images o boiling low in exit region at CHF and during CHF transient or G = 335 kg/m s and G = 650 kg/m s. Fig. 6 shows boiling curves or G = 507 and 804 kg/m s measured by the most downstream thermocouple o the heated wall, where CHF was detected irst or these two mass velocities. Shown No Heat Inlet Region Inlet Region G = 335 kg/m s Inlet Region G = 650 kg/m s 50% CHF Middle Region Middle Region 95% CHF Exit Region Exit Region G = 650 kg/m s, x i = , i = 0.64 x i = , i = 0.54 x i = , i = 0.64 Fig. 4. Images o boiling low depicting eects o increasing heat lux or inlet region with G = 650 kg/m s, and variations in boiling behavior with mass velocity or inlet, middle and exit regions at 95% CHF.

7 460 C.R. Kharangate et al. / International Journal o Heat and Mass Transer 55 (0) Heat Flux ( W/cm ) G = 804 kg/m s G = 507 kg/m s T w - T sat ( º C) Fig. 6. Boiling curves or G = 507 kg/m s(x i = 0.05) and G = 804 kg/m s(x i = 0.0). are both transient and steady state data to illustrate both how the steady-state boiling curve data are reached ollowing each power increment, and the clear wall temperature excursion ollowing the attainment o CHF. Fig. 6 shows the higher mass velocity nets an increase in CHF value. Fig. 7 and show the variation o CHF with inlet quality, x i, and exit quality, x o, respectively. A broader range o quality is possible at low mass velocities. Achieving the same quality range is more diicult at higher mass velocities because o an appreciable increase in pressure drop across the low channel. Both igures show CHF mostly increasing, albeit slightly, with increases in x i or x o. Despite the reduced liquid content with increasing quality, this CHF trend can be explained by the higher quality values increasing the liquid velocity. Both igures show the expected trend o CHF increasing with increasing mass velocity. Void raction could be measured only or relatively low mass velocities in the range o G = kg/m s. This is not possible or higher mass velocities because o the aorementioned lack o phase separation. Fig. 7(c) shows the variation o CHF with inlet void raction, a i, or dierent mass velocities. For each mass velocity, CHF generally increases with increasing a i because o the aorementioned increase in liquid velocity. Fig. 7(d) shows CHF increases monotonically with increasing mass velocity or a given value o exit quality. Experiments with mass velocities below 340 kg/m s show dierences in interacial behavior during the CHF transient rom those observed with higher mass velocities. First, three clearly separated layers are observed at the inlet to the heated portion o the channel below 340 kg/m s. This behavior is ar less obvious at high mass velocities because o the thinning o the vapor wall layer. Second, CHF below 340 W/m s is initiated at the most upstream thermocouple, while CHF or higher mass commences at the most downstream thermocouple. Fig. 8 and show temporal temperature records measured by the ive thermocouples embedded along the heated wall during the CHF transient up to and including the instant the electrical power input to the heated wall was cuto to prevent any physical damage caused by the CHF temperature excursions. In these plots, T denotes the most upstream temperature and T 5 the most downstream. For a low mass velocity o G = 77.6 kg/m s(x i = 0.5), Fig. 8 shows CHF is detected irst by the most upstream thermocouple T ollowed by T. On the other hand, Fig. 8 shows CHF or a relatively high mass velocity o G = 583 kg/m s(x i = 0.049) is detected irst by the most downstream thermocouple T 5 ollowed by T 4. These transient CHF trends provide valuable insight into the CHF mechanism. Prior studies o low boiling CHF or zero inlet void conditions show ample evidence that CHF is preceded by the ormation o a wavy vapor layer along the heated wall, which permits liquid access to the wall only in wetting ronts, where the troughs o the wavy vapor layer interace make contact with the wall [7 9,5,6 8]. The same studies show that CHF is triggered by lit-o o a wetting ront, preventing liquid access to the wall. With this lit-o, heat rom the wall is now concentrated in a ewer number o wetting ronts, which accelerates the lit-o o these wetting ronts as well. Discussed later in this study is how the interacial lit-o mechanism may explain both the CHF mechanism and dierences in the location o CHF commencement. Fig. 9 and show pressure drop measured across the heated wall channel at CHF-. This pressure drop includes a 3- mm region between the upstream pressure tap and upstream edge o the heated wall, and a second 3-mm region between the downstream edge o the heated section and downstream pressure tap. It should be noted that the cross-section o the low channel is uniorm throughout, including both the development low region upstream o the heated length and the downstream region. Fig. 9 shows the pressure drop increases monotonically with increasing mass velocity or a ixed inlet quality. Notice how higher quality values increase pressure drop, which can be explained by both the rictional and accelerational components o pressure drop increasing with increasing quality. Fig. 9 shows that increasing inlet void raction or a ixed mass velocity precipitates a monotonic increase in pressure drop; this increase is more pronounced at higher mass velocities. 5. Separated low model A new type o separated low model is proposed as a oundation or CHF prediction. Fig. 0 shows a schematic o horizontal low with inite inlet void based on conditions observed at CHF-. Because o buoyancy eects, the low upstream o the heated wall is assumed to consist o a vapor layer residing above a liquid layer. A third vapor layer begins to orm at the leading edge o the heated wall, resulting in a separated three-layer low. The mass low rate o the lower vapor layer adjacent to the heated wall grows along the heated wall. Slip low assumptions are adopted in the model, meaning each o the three layers is characterized by a uniorm velocity while allowing or dierences among velocities o the three layers. Pressure is assumed uniorm across the channel s cross-sectional area. In this model, mass, momentum and energy conservation laws are applied to a control volume o length Dz o the low channel starting with the upstream edge o the heated wall. The ollowing equations are used to relate low quality, velocity and void raction or the top vapor layer o thickness d, lower vapor layer o thickness d, and intermediate liquid layer o thickness H d d, respectively. x ¼ U ga g GA x ¼ U ga g GA and ð x x Þ¼ q U A GA ¼ U ga ; ðþ G ¼ U ga ; ðþ G ¼ q U ð a a Þ : ð3þ G Conservation o mass or the combined low gives dw dz ¼ 0; which implies both W and G are constant. Because the entire low is saturated, there is no temperature gradient between the top vapor layer and middle liquid layer. ð4þ

8 C.R. Kharangate et al. / International Journal o Heat and Mass Transer 55 (0) CHF (W/cm ) CHF (W/cm ) G = 85 kg/m s G = 335 kg/m s G = 500 kg/m s G = 650 kg/m s G = 790 kg/m s G = 970 kg/m s G = 50 kg/m s G = 300 kg/m s G = 490 kg/m s G = 65 kg/m s G = 85 kg/m s G = 335 kg/m s G = 500 kg/m s G = 650 kg/m s G = 790 kg/m s G = 970 kg/m s G = 50 kg/m s G = 300 kg/m s G = 490 kg/m s G = 65 kg/m s x i x o CHF (W/cm ) G = 85 kg/m s 4 G = 335 kg/m s G = 500 kg/m s G = 650 kg/m s G = 790 kg/m s i (c) CHF (W/cm ) G (kg/m s) (d) x o = 0.05 x o = 0.08 x o = 0. x o = 0.4 x o = 0.8 x o = 0.5 Fig. 7. Variation o CHF with inlet low quality, exit low quality, (c) inlet void raction and (d) mass velocity. Thereore, any minor mass transer along the interace is assumed negligible and the mass low rate o the top vapor layer, x W, constant, which implies x is also constant. The growth o the lower vapor layer is accounted or entirely by evaporation o the middle liquid layer. Conservation o mass yields the ollowing relation or the rate o liquid evaporation along the lower vapor liquid interace. W 0 g ¼ GA dx dz : Conservation o momentum or control volumes o length Dz encompassing the top vapor layer, middle liquid layer, and lower vapor layer, yields, respectively, $ % G d x dp ¼ a dz a dz s w;gp w;g A G d dz $ % ð x x Þ q ð a a Þ ð5þ s ip i A ; ð6þ þ W 0 g u i ¼ ð a a Þ dp dz s w; P w; A s ip i A s ip i A ; ð7þ and $ % G d x W dz a 0 g u dp i ¼ a dz s w;gp w;g A s ip i A : ð8þ In the above equations, P is the pressure, s w,g, s w, and s w,g the wall shear stresses or the top vapor layer, middle liquid layer and bottom vapor layer, respectively, P w,g, P w,, and P w,g the perimeter o the top vapor layer, middle liquid layer and lower vapor layer, respectively, in contact with the channel walls, and P i and P i the contact perimeters between the top vapor layer and middle liquid layer and between the middle liquid layer and bottom vapor layer, respectively. The ±sign in the interacial shear terms is intended to allow or any variations in the direction o the shear stress, depending on local velocity dierences between the three layers. The shear stress direction can be traced on a local basis rom numerical solution o Eqs. (6) (8), combined with mass and energy conservation. Because the vapor generated at the wall is ejected normal to the wall, it will have no initial stream-wise velocity [9] and, as such, does not contribute stream-wise momentum to the control volume. Thereore, the interacial momentum terms in Eqs. (7) and (8) are neglected. Applying the relations a = d /H and a = d /H to deine riction perimeters, and recalling that dx /dz = 0, Eqs. (6) (8) can be expressed, respectively, as

9 46 C.R. Kharangate et al. / International Journal o Heat and Mass Transer 55 (0) Temperature ( o C) G = 77.6 kg/m s x i = 0.5 Power Cuto Time (s) T T T 3 T 4 T 5 Pressure Drop (Pa) 8,000 6,000 4,000,000 0, Mass Velocity (kg/m s) x i = 0.0 x i = 0.05 x i = 0.09 x i = 0.5 Temperatures ( o C) T 5 T 4 T T T 3 Pressure Drop (Pa) G = 85 kg/m s G = 335 kg/m s G = 500 kg/m s G = 690 kg/m s G = 750 kg/m s G x da G a dz q g G q and 00 G = 583 kg/m s x i = Power 90 Cuto Time (s) Fig. 8. Temporal records o heated wall thermocouples during CHF transient or G = 77.6 kg/m s(x i = 0.5) and G = 583 kg/m s(x i = 0.049). x dq g dp a dp dz ¼ a dp dz s w;g H þ w a s i H ; $ % ð x x Þ dx ð a a Þ dz þ ð x x Þ da ð a a Þ dz þ ð x x Þ da ð a a Þ dz G ð x x Þ dq dp q ð a a Þ dp dz ¼ ð a a Þ dp dz s w; s i H s i H G x dx a dz x da G x dq g dp dz a dp dz ¼ a dp dz s w;g a q g The wall shear stress or each phase is deined as s w;k ¼ q k U k k; H þ w a ð9þ w ð a a Þ s i H : ð0þ ðþ ðþ where k = or g, depending on the phase being modeled. The riction actor in Eq. () is obtained rom the ollowing relation by Bhatti and Shah [9], k ¼ C þ C C ¼ C Re =C þ 3 =C3 ; ð3þ q D;k k U k D k l k where C =0, C = 6 and C 3 = or laminar low (Re D,k 6 00), C = , C = and C 3 = /3 or transitional low (00 < Re D,k ), and C = 0.008, C = 0.43 and C 3 = 3.54 or turbulent low (Re D,k > 4000). The diameter in Eq. (3) is deined as D k =4A k /P k =H k w/(h k + w). The two interacial shear stresses are determined according to the relations s i ¼ C ;i ðu g U Þ ð4þ and Fig. 9. Variation o measured pressure drop with mass velocity or dierent inlet qualities, and inlet void raction or dierent mass velocities. s i ¼ C ;i ðu g U Þ ð5þ where C,i is the interacial riction coeicient. Galloway and Mudawar [8] examined several models to determine C,i and recommended a constant value o 0.5 or a wavy vapor liquid interace. Applying energy conservation to a control volume o length Dz encompassing the entire cross-sectional area o the channel yields dx dz ¼ dx dz ¼ q00 P h Wh g ¼ q00 w Wh g : ð6þ Eqs. () (3), (9) () and (6) and are solved simultaneously using a ourth-order Runge Kutta numerical scheme along the channel using saturated properties based on local pressure. This yields values or pressure, qualities, void ractions and velocities o three separated layers or every Dz axial increment rom the upstream edge o the heated wall to the downstream edge. The main inputs required or the model, which are deined at the leading edge o the heated wall, are mass velocity, inlet pressure, inlet quality x i i

10 C.R. Kharangate et al. / International Journal o Heat and Mass Transer 55 (0) g e w Vapor Flow Vapor iquid H H iquid Vapor Vapor 0 z q q Cross-section at z.0 Ratio o Separated ayer Thickness to Channel Height G = 780 kg/m s P i = 5 kpa x i = i = 0.8 q = 3.34 W/cm z (mm) Total vapor void raction: + Fig. 0. Schematic o separated three-layer model. Model predictions o void raction. (which is equal to inlet vapor quality x i o the top vapor layer), inlet height d i o the top vapor layer, and wall heat lux q 00. Fig. 0 shows thickness variations o the three separated layers along the heated portion o the low channel or G = 780 kg/ m s, P i = 5 kpa, x i = 0.085, a i = 0.8, and q 00 = 3.34 W/cm. The thicknesses are represented by void ractions a or the top vapor layer, a a or the middle liquid layer, and a or the lower vapor layer. The model shows the lower layer growing in thickness axially, while the top vapor layer is squeezed along because o the axially increasing shear orces. Also shown in Fig. 0 is the total void raction, a + a, increasing gradually along the heated length because o the relatively ast thickening o the lower vapor layer. As discussed earlier in conjunction with the low visualization results, the separated three-layer behavior is clearly recognizable near in the inlet region o the heated wall, but diicult to track or the middle and exit regions. Recall that the model tackles all the orces acting axially on the control volumes used. The model does not account or the transverse buoyancy orces responsible or any potential downstream mixing between the layers. It can thereore be concluded that the model s greatest value is in predicting interacial behavior in the upstream region. As shown in Fig. 8, this is the region where CHF is detected irst or relatively low mass velocities, rendering the model eective at predicting CHF or these conditions. Even or high mass velocities, the Interacial it-o Model is based on upstream development o the wall vapor layer, which is where the interacial wavelength is established [9]. The present separated low model is used to predict pressure drop over a mass velocity range o kg/m s. Predictions are compared to experimental data rom the present study based on input measured values o G, P i, x i, a i and q 00. As indicated in Table, the separated low model predicts the present data with a mean absolute error o 56.9%, where MAE ¼ M X jdppred DP exp j DP exp 00% ð7þ This error can be traced to the aorementioned downstream buoyancy and mixing eects not accounted or in the model. Nonetheless, as discussed later, the greatest value o the model is in predicting the upstream development o the wall vapor layer. The present pressure drop data are also compared to predictions o the Homogenous Equilibrium Model (HEM). As shown in Table, both constant two-phase riction actors [30 34] and two-phase mixture viscosity models [35 4] are used to predict the two-phase rictional pressure gradient. Predictions are provided or two dierent constant riction actors, tp = and 0.005, and seven dierent two-phase viscosity models. All HEM

11 Table Comparison o measured pressure drop to predictions o three-layer separated low model and homogeneous equilibrium model (HEM) using dierent two-phase riction actors and viscosity models. Three-layer separated low model MAE = 56.9% Homogeneous equilibrium model (HEM) n dp dz ¼ dp dz þ o dp A dz F dp dz A ¼ G dx v g dz dp dz ¼ F D h tpt G þ x t g t ; D h ¼ 4A P ¼ 4Hw ðhþwþ Constant riction actor method Author (s) Applications tp MAE [%] ewis and Robertson [30], and Markson et al. [3] High pressure steam-water boilers Bottomley [3], Benjamin and Miller [33], and Allen [34] ow pressure lashing steam-water lows Two-phase mixture viscosity method tp =6Re tp or Re tp < 000 tp ¼ 0:079Re 0:5 tp or Re tp < 0,000 tp ¼ 0:046Re 0: tp or Re tp P 0,000 Re tp = GD h /l tp Author (s) Equation MAE [%] McAdams et al. [35] ltp ¼ l x þ x g l Akers et al. [36] l tp ¼ l 0: ð xþþx tg t Cicchitti et al. [37] l tp = xl g + ( x)l 7.6 Owens [38] l tp = l 5.43 Dukler et al. [39] l tp ¼ xtgl þð xþt g l xtg þð xþt 5.40 Beattie and Whalley [40] in et al. [4] l tp ¼ xl g þð xþð þ :5xÞl x ¼ xt g t þ xt g l l tp ¼ l g l g þx :4 ðl l g Þ C.R. Kharangate et al. / International Journal o Heat and Mass Transer 55 (0)

12 C.R. Kharangate et al. / International Journal o Heat and Mass Transer 55 (0) Table Correlations or low boiling CHF in rectangular and circular channels, and corresponding MAE in predicting present CHF data. Author (s) Equation Comments MAE [%] Bowring [4] q 00 m ¼ A þ 0:5D h;egdh sub;i C þ Circular channel Vertical uplow Uniormly heated 37.6 Dh sub;i ¼ h h i ; A ¼ :37ðD h;egh g =4ÞF :0 þ 0:043F D 0:5 h;e G 0:077F 3 D h;e G C ¼ :0 þ 0:347F 4 ðg=356þ n n ¼ :0 0:5P R ; P R ¼ 0:45P o ; P o in MPa F ¼ n o :97 P8:94 R exp½0:89ð:0 P R ÞŠ þ 0:97 :309F F ¼ P :36 R exp½:444ð:0 P R ÞŠ þ 0:309 F 3 ¼ n o R exp½6:658ð:0 P R ÞŠ þ 0:667 :667 P7:03 F 4 ¼ F 3 P :649 R Katto [43] q 00 m ¼ q00 m0 :0 þ K Dh sub;i h g q 00 m0 ¼ 0:5ðGh gþ =D h;e q 00 m0 ¼ CðGh gþwe 0:043 ; We ¼ G =D h;e rq q 00 m03 ¼ 0:5ðGh gþ! 0:33 We q þ 0:0077=D h;e q 00 m03 ¼ 0:6ðGh gþ! 0:33 We q ð=d h;e Þ 0:7 þ 0:0077=D h;e Rectangular channel Vertical low For water: One-sided heated wall 6 < /D h,e < 500 or P = MPa and 0.47 < /D h,e < 6.0 or P = 0 kpa For R-3: Two-sided heated walls /D h,e =5 or P = 0 47 kpa 55.8 For =D h;e < 50; C ¼ 0:5 For =D h;e > 50; C ¼ 0:34 K ¼ K ¼ 0:6 CWe 0:043 K 3 ¼ 0:5556ð0:0308 þ D h;e=þ ð =q Þ 0:33 We =3 When q 00 m0 < q00 m0 ; q00 m0 ¼ q00 m0 ; K ¼ K When q 00 m0 > q00 m0 ; i q00 m0 < q00 m03 ; q00 pm0 ¼ q00 m0 ; K ¼ K I q 00 m0 > q00 m03 ; i q00 m03 < q00 m04 ; q00 m0 ¼ q00 m03 ; K ¼ K 3 I q 00 m03 > q00 m04 ; q00 m0 ¼ q00 m04 Mishima and Ishii [44] q 00 m ¼ A h g 0: A h C 0 q iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii gðq ÞD h;e For rectangular channels : siiiii C 0 ¼ :35 0:35 q For round tubes : siiiii C 0 ¼ : 0: q þ Dh sub;i G h g Circular internally heated annulus Vertical uplow For water: D i = m D o = m h = m P = 0 kpa G = kg/m s Dh sub,i = kj/kg 5. Katto and Ohno [45] Circular channel Vertical uplow 6.6 (continued on next page)

13 466 C.R. Kharangate et al. / International Journal o Heat and Mass Transer 55 (0) Table (continued) Author (s) Equation Comments MAE [%] q 00 m ¼ q00 m0 :0 þ K Dh sub;in h g q 00 m0 ¼ CðGh gþwe 0:043 =D h;e! 0:33 q 00 m0 ¼ 0:ðGh gþ We q =3 þ 0:003=D h;e! 0:33 q 00 m03 ¼ 0:098ðGh gþ We q 0:433 ð=d h;e Þ 0:7 þ 0:003=D h;e Uniormly heated For water: 0 < /D h,e < 500 P = 0 00 bar For R-: D h,e = 0.0 m =m P = MPa G = 0 00 kg/m s Dh sub,i = kj/kg q 00 m04 ¼ 0:0384ðGh gþ q! 0:6 We 0:73 þ 0:8We 0:33 ð=d h;e Þ q 00 m05 ¼ 0:34ðGh gþ! 0:53 We q ð=d h;e Þ 0:7 þ 0:003=D h;e For =D h;e < 50; C ¼ 0:5 For 50 6 =D h;e 6 50; C ¼ 0:5 þ 0:0009ð=D h;e 50Þ For 50 < =D h;e ; C ¼ 0:34 K ¼ 0:6 CWe 0:043 K ¼ 0:8333ð0:04 þ D h;e=þ ð =q Þ 0:33 We =3 K 3 ¼ :ð:5we 0:33 þ D h;e =Þ ð =q Þ 0:6 We 0:73 For =q < 0:5 : When q 00 m0 < q00 m0 ; q00 m0 ¼ q00 m0 When q 00 m0 > q00 m0 ; i q00 m0 < q00 m03 ; q00 m0 ¼ q00 m0 ; i q 00 m0 > q00 m03 ; q00 m0 ¼ q00 m03 When K > K ; K ¼ K ; when K < K ; K ¼ K For =q > 0:5 : When q 00 m0 < q00 m05 ; q00 m0 ¼ q00 m0 When q 00 m0 > q00 m05 ; i q00 m05 > q00 m04 ; q00 m0 ¼ q00 m05 ; i q 00 m05 < q00 m04 ; q00 m0 ¼ q00 m04 When K > K ; K ¼ K ; when K < K ; i K < K 3 ; K ¼ K ; i K > K 3 ; K ¼ K 3 Sudo et al. [46] Oh and Englert [47] Rectangular channel " siiiiiiiiiiiiiiiiiiiiii# Vertical uplow 0:945 q 00 m ¼ 0:005h r gg 0:6 ðq Þg ðq Þg q 00 m ¼ A P h;e h g 0:458 :0 þ Dh qiiiiiiiiiiiiiiiiiiiiiiiiiiiiii sub;i G þ :4 kq h g gðq Þ g siiiiiiiiiiiiiiiiiiiiii r k ¼ ðq Þg Two-sided heating For water: /D h,e = 70 P = kpa G = kg/m s Rectangular channel Vertical uplow Uniormly heated For water: P = 0 85 kpa G = kg/m s D T sub,i = 5 7 C methods are shown underpredicting the pressure drop data, with MAEs ranging rom 5.43 to 7.68%. With a MAE o 5.43%, the viscosity model by Owens [38] provides the best predictions among all HEM methods, ollowed by the viscosity model o Cicchitti et al. [37], which has a MAE o 7.6%. 6. CHF predictions The present CHF data are compared to predictions o both prior empirical correlations [4 47] and the Interacial it-o Model. Table details three correlations that were developed or low

14 C.R. Kharangate et al. / International Journal o Heat and Mass Transer 55 (0) CHF (predicted) [W/cm ] boiling CHF in rectangular channels and three other correlations or circular channels. Since some o these correlations are capable o predicting both subcooled and saturated inlet conditions, the subcooling term in these correlations is set to zero when comparing predictions to the present data. Notice that some o these correlations were developed or water alone, others are applicable to other luids as well. Notice in Table that the previous CHF correlations are based on dierent wall heating conditions, some involving heating o only a portion o the circumerence, others ull circumerential heating. The present rectangular channel coniguration involves heating along a heated perimeter equal to one width (w) o the rectangular channel. Thereore, the channel diameter in these correlations is replaced with an equivalent heated diameter deined as D h;e ¼ 4A w ¼ 4H: ð8þ Fig. compares the predictions o the six correlations to the present data. The accuracy o individual correlations is ascertained using mean absolute error, which is deined as MAE ¼ M Bowring (97) (37.6% MAE) Katto (98) (55.8% MAE) Mishima & Ishii (98) (5.% MAE) Katto & Ohno (984) (6.6% MAE) Sudo et al. (985) (7.40% MAE) Note: Oh & Englert s (993) data not shown because o large MAE (55.0%) Interacial it-o Model (.63%) +30 % CHF (experimental) [W/cm ] -30 % Fig.. Comparison o present FC-7 CHF data with predictions o prior CHF correlations and interacial lit-o model. X jchfpred CHF exp j CHF exp 00% ð9þ With a MAE o 5.%, Mishima and Ishii s correlation [44] shows the best agreement with the present data. It is ollowed in accuracy by Katto and Ohno s correlation [45], with a MAE o 6.6%. While Katto s correlation [43] is applicable to rectangular channels, its predictions are inerior to those o Katto and Ohno, with a MAE o 55.8% compared to 6.6%. Despite its moderate MAE o 37.6%, the Bowring correlation [4] does not capture the correct trend o CHF with mass velocity. The correlation by Sudo et al. [46] has a relatively large MAE o 7.40%. The correlation by Oh and Englert [47] is the least accurate o the correlations tested, with MAE o 55.0%. It is important to emphasize that poor predictive capability o a given correlation is not necessarily an assessment o the general accuracy o the correlation, but its unsuitability to the working luid, low geometry and operating conditions o the present study. The present CHF data are also compared to predictions o the Interacial it-o Model. This model has shown remarkable accuracy in predicting low boiling CHF with zero inlet void raction [7 9,5,6 8]. In the original model, the low arrives to the heated portion o the channel in saturated or subcooled liquid state, and a wavy vapor layer begins to develop along the heated wall at CHF-. This model is built on the observation that partial wetting o the wall at CHF-is possible only in wetting ronts corresponding to the troughs o the wavy vapor layer interace. CHF is described as the result o liting o wetting ronts rom the wall due to intense vapor eusion. Zhang et al. [5] showed that CHF or zero inlet subcooling can be predicted according to the ollowing relation, q 00 m ¼ b h g " # = ; ð0þ 4prd sinðbpþ! bk c z where b = 0., d is the thickness o the vapor layer, and k c the critical wavelength o instability o the vapor layer interace; both d and k c are calculated at z = z 0 + k c (z ), where z 0 is the location where the velocity o the vapor layer just exceed that o liquid at the same axial location. The critical wavelength is given by [5] viiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii p k c g ðu g U Þ u þ t4 r q 00 þ q 00 g ¼ q00 q00 where q 00 ¼ q coth p q 00 g ¼ coth p k c H g 3 g ðu g U Þ 5 r q 00 þ q 00 g q 00 q00 þ ðq Þg ; r ðþ H ; ðþ k c ; ð3þ and H and H g are the thicknesses o the liquid and vapor layers, respectively. In the original Interacial it-o Model, a two-layer separated low model is used to determine the axial variations o U g, U and d, rom which the values o d and k c are calculated at z to determine CHF according to Eq. (0). Unlike the original two-layer separated low or which the original Interacial it-o Model is developed, the present study involves three separated layers because o the inite inlet void raction. To modiy the model to the conditions o the present study, the instability criteria are applied to the interace between the middle liquid layer and bottom vapor layer. Thereore, the vapor layer thickness, d, is Eq. (0) is substituted by d, and the vapor velocity, U g, in Eq. () by U g. Furthermore, because the present separated low model predicts U g always exceeding U, z 0 is set equal to zero, hence both d and k c in Eq. (0) are evaluated at z = k c (z ). The mean liquid and vapor layer thicknesses. H and H g. in Eqs. () and (3) are given, respectively, by H ¼ð a a ÞH and H g ¼ a H: ð4þ ð5þ Fig. shows the modiied Interacial it-o Model is equally successul at predicting CHF or low boiling in a horizontal channel with inite inlet void raction, evidenced by a MAE o.63%, which is superior to those o the CHF correlations included in the same igure. This demonstrates the validity o the proposed CHF mechanism or the high mass lux conditions that could not be captured with high resolution using video imaging. 7. Conclusions This study explored saturated low boiling CHF in a rectangular horizontal channel with an upward-acing heated wall. The inluence o inlet vapor void on interacial behavior at heat luxes up to CHF as well during the CHF transient was examined with the