THE INFLUENCE FACTORS ON HEAT TRANSFER PERFORMANCE OF LOOP THERMOSYPHON SYSTEM. Li-Chieh Hsu, Guo-Wei Wong and Kung-Ting Chen

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1 THE INFLUENCE FACTORS ON HEAT TRANSFER PERFORMANCE OF LOOP THERMOSYPHON SYSTEM Li-Chieh Hsu, Guo-Wei Wong and Kung-Ting Chen Department of Mechanical Engineering, National Yunlin University of Science and Technology, Douliou, Yunlin, Taiwan, R.O.C. ICETI 2015, SG5007_SCI No.16-CSME-60, E.I.C.Accession 3946 ABSTRACT The influence factors on the heat transfer performance of a loop thermosyphon system, a passive cooling device, are studied systematically. The parameters investigated include types of enhanced boiling structure, the depth to width ratio of enhanced boiling structures, the gap of evaporator, the condenser height and the inclination of evaporator. The results show the depth to width ratio and the condenser height has positive influences on the heat transfer performance. An optimal channel gap of evaporator exists and possesses better heat transfer performance. The inclination effect of evaporator may not be favorable to heat transfer. Among those, the horizontal and 90 inclination of evaporator has better cooling performance. Keywords: thermosyphon; boiling; enhanced structure. FACTEURS D INFLUENCE SUR LA PERFORMANCE DE TRANSFERT DE CHALEUR D UN SYSTÈME THERMOSIPHON EN BOUCLE RÉSUMÉ Les facteurs qui ont une influence sur la performance du transfert de chaleur d un système thermosiphon en boucle, un dispositif de refroidissement passif, sont examinés dans cette recherche. Parmi les paramètres investigués se trouvent les types de structures renforcées d ébullition, le ratio de la profondeur et la largeur des structures ; l écart de l évaporateur, la hauteur du condenseur, et l inclinaison de l évaporateur. Les résultats démontrent que le ratio de la profondeur par rapport à la largeur, et la hauteur du condenseur, ont une influence positive sur la performance du transfert de chaleur. Il y a une taille optimale de l espace entre les canaux de l évaporateur qui donne une meilleure performance de transfert de chaleur. L effet d inclinaison de l évaporateur peut ne pas être favorable à ce transfert. Ainsi, l inclinaison horizontale de 90 de l évaporateur offre une meilleure performance de refroidissement. Mots-clés : thermosiphon; ébullition; structure renforcée. Transactions of the Canadian Society for Mechanical Engineering, Vol. 40, No. 5,

2 1. NOMENCLATURE H the elevation of condenser relative to evaporator P f frictional pressure drop P ftp two-phase frictional pressure drop P a two-phase accelerational pressure drop P G two-phase gravitational pressure drop G mass velocity (kg/m 2 s) A c area of cross section D hydraulic diameter v f specific volume of liquid v f g difference of specifc volume between saturated liquid and saturated vapor X out vapor quality at the outlet of evaporator L length of conduit two-phase flow friction factor f TP Greek symbols ρ density (kg/m 3 ) Subscripts g gravity 2. INTRODUCTION Two-phase closed thermosyphons have received considerable attention in many industrial and energy applications from earlier application on electronics cooling to current solar heating system for buildings. The properties of low cost, simple fabrication and high thermal conductivity make thermosyphon still be a promising cooling solution. The superiority pumpless loop thermosyphon to pool boiling thermosyphon had been proved by Mukherjee and Mudawar [1] by higher critical heat flux (CHF) of pumpless loop type than that of pool boiling type. Their results show the CHF increases as the evaporator gap decreases. The evaporator gap is the distance between cover of evaporator and the tip of enhanced boiling structures. However, the CHF may decreases if the gap is smaller than the crossover gap. The study further compared the CHF of three types of boiling surface structures which are microchannel (channel depth < 1 mm), minichannel (channel depth > 3 mm) and flat. Among those, the CHF of microchannel is highest. The CHF of minichannel is better than that of flat surface. These results raise our interest in investigating the more parameters which affect the heat transfer performance of loop thermosyphon. Besides the enhanced boiling surface structure and evaporator gap, the condenser height and the degree of inclination of evaporator are also studied in this research. The effect of condenser height on flow rate and the amount of limiting heat flux of loop thermosyphon were reported by Garrity et al. [2]. Their results show the largest flow rate occurs once the condenser altitude is highest which results in the larger pressure difference between riser and downcomer. At low heat loads, the flow rate is increased as the heat load increases. However, at higher heat loads, the flow rate is decreased as the heat load increases. The reason is that the increasing vapor quality causes significant frictional pressure drop. For a condenser height, the flow rate is increased as the heat load increases, if heat load is less than a limiting heat flux. Once heat load is larger than the limiting heat flux, the system becomes unstable. The reason is that the frictional pressure drop is increased obviously which leads to decreasing flow rate. Kandilikar and Balasubramanian [3] performed two-phase flow visualization as well as heat transfer measurements in a µm parallel microchannel with 332 µm hydraulic diameter by using water. Their 948 Transactions of the Canadian Society for Mechanical Engineering, Vol. 40, No. 5, 2016

3 results showed that the heat transfer coefficients for horizontal and upward vertical are the same but the heat transfer coefficient for vertical downward arrangement is impaired by 30 40%. Zhang [4] conducted two-phase boiling experiments of FC-72 flow in a microchannel heat sink with the channel dimensions of 0.2 mm (W) 2 mm (H) 15 mm (L), and presented results for three different orientations: vertical upflow (VU), vertical downflow (VD) and horizontal flow with horizontal facing (HH). Their results showed that comparable thermal resistance amid vertical downflow and horizontal arrangement, and the thermal resistance of vertical upflow is slightly lower ( 5%). Wang et al. [5] examined the effect of inclination on the convective boiling heat transfer characteristics of the dielectric fluid HFE-7100 within an 825 m multiport microchannel heat sink. The inclinations spans from 90 (vertical downward) to 90 (vertical upward), and a flow visualization is also conducted in their study. They found that the heat transfer coefficient for the vertical upward and horizontal is comparable, and the heat transfer coefficient for 45 upward considerably exceeded other configurations. In summary of the foregoing discussion, it appears that the influence of inclination on the heat transfer performance of a microchannel heat sink is not conclusive. Although, the previous studies of inclination of evaporator effect on heat transfer coefficient is limited to evaporator itself rather than thermosyphon system, those referable conclusions drove our attention to investigate on this effect on thermosyphon. Hence, the effects of enhanced boiling surface structure, evaporator gap, the condenser height and the inclined angle of evaporator on the heat transfer performance of themosyphon system will be investigated in this study. 3. EXPERIMENTAL SETUP 3.1. Apparatus The experimental loop thermosyphon system includes evaporator, condenser, tank with breather and valves as show in Fig. 1(a) Setup Procedure As shown in Fig. 1(a), first, the working fluid, FC-72, is filled into tank 70higher than the condenser. On the top of tank, a breather is set to make sure the tank is open to ambient. The fluid can be filled into the circulation system by gravity and ambient pressure. During the filling process, the residual air inside the loop floats upward to the tank and is ventilated through the breather. Once the necessary filling rate is reached at 80minor adjustment on filling rate of working fluid. Four thermocouples are employed in inlet and outlet of evaporator and condenser, respectively. Three locations with thermocouples are set beneath the boiling structure to obtain the mean heater surface temperature. Further, the channel surface temperature can be derived by Fourier s heat conduction relation based on two vertical measure points with thermocouples inserted copper boiling structure as shown in Fig. 2(c). A flow meter is located in front of inlet of evaporator to measure the circulation mass flow rate of working fluid Evaporator Design The evaporator includes copper made boiling surface in the bottom and an acrylic transparent cover on top as shown in Figs. 2(a) and. These two parts are assembled to form a mini channel where working fluid can flow through. The diverging area and the converging area are set in the inlet and outlet of the channel, respectively. The evaporator gap is the defined as the distance from the transparent cover to the boiling surface itself for the flat surface or to the tips of the fins for the enhanced surfaces as shown in Fig. 2(d). There are various enhanced boiling structures as shown in Table 1. Every type of enhanced area size is mm 2. The heater is attached beneath the boiling structure and insulated well to enforce the heat only can be conducted upward to the boiling structures. On the top of the transparent cover, a pressure Transactions of the Canadian Society for Mechanical Engineering, Vol. 40, No. 5,

4 (a) Fig. 1. (a) Experiment apparatus; schematic of loop thermosyphon system. transducer is employed to record the evaporator pressure. Further the corresponding saturated temperature can be obtained with this pressure magnitude Experimental Parameters First of all, various enhanced boiling structures have been investigated to study the effects on the heat transfer performance. Secondly, the effect of channel gap is studied with a selected enhanced boiling structure of evaporator. As known the gravity is one of important driving force of circulation, the height of condense respect to evaporator is investigated for its influence. Further, to consider more operational conditions, the effect of inclination of evaporator is also studied. 4. DRIVING FORCE The pressure head built by the elevation of condenser relative to evaporator is used to overcome the pressure drop caused by friction, two-phase flow acceleration and gravitation in the sections of downcomer evaporator, riser and condenser as shown in Eq. (1): ρgh = P f,4 1 + P ftp,1 2 + P a,1 2 + P G,2 3 + P ftp,2 3 + P ftp,3 4 + P a,3 4 (1) P f,4 1 = f ( ) ( ) H ρ f ṁ 2, f = 64 (2) D 2 ρ f A c Re D where A c is the area of cross section. The two-phase frictional pressure drop is based on the homogeneous model [6] as shown in Eq. (3). The value of friction factor, f TP, is suggested by the report of Mudawar [1]. P ftp,1 2 = 2 f ( TP D G2 v f L X ) out v f g, f TP = (3) 2 v f 950 Transactions of the Canadian Society for Mechanical Engineering, Vol. 40, No. 5, 2016

5 (a) (c) (d) Fig. 2. Evaporator. (a) Explosion diagram, top view of assembly, (c) two thermal measure points inside copper structure along vertical direction, (d) schematic definition of evaporator gap. where G = ṁ/a c. Two-phase accelerational pressure drop across the evaporator as P a,1 2 = G 2 X out v f g (4) The vapor quality is unchanged due to the adiabatic flow from the outlet of evaporator to inlet of condenser. Therefore, the two-phase gravitational pressure drop and frictional pressure drop are shown in Eqs. (5) and (6), respectively. 1 P G,2 3 = gh (5) v f + X out v f g P ftp,2 3 = 2 f TPG 2 D H(v f + X out v f g ) (6) The two-phase frictional pressure drop in condenser has the same model with the evaporator as shown in Eq. (3). The major difference in magnitude is dependent on the length of conduit. P ftp,3 4 = 2 f ( TP D G2 v f L ) 2 X v f g out (7) v f Although the two-phase accelerational pressure drop across the condenser has the same model as that of evaporator, the negative pressure drop contribute the partial pressure recovery as P a,3 4 = G 2 X out v f g (8) Transactions of the Canadian Society for Mechanical Engineering, Vol. 40, No. 5,

6 Table 1. Various enhanced boiling structures with different depth to width ratios. The mechanical equilibrium equation of a loop thermosyphon is obtained after all items on the right-hand side of Eq. (1) are substituted by Eqs. (2 8). 5. RESULTS AND DISCUSSIONS 5.1. Boiling Structure Effect The enhanced boiling structure of micro fin arrays was experimentally studied by Zhang [7] whose report pointed out that the boiling heat transfer is significantly enhanced by the fin arrays of the heating surface Structure types Three types of boiling surface structures are investigated. Each of them has four kinds of depth to width ratio. The boiling surface engraved by machining to be rectangular concave minichannels perpendicular to main flow direction is named type-a. The minichannels parallel to main flow direction is named type-b. The boiling surface engraved to be square-pins-array isotropic for main flow direction is named type-c. All types of structure are listed in Table Depth to width ratio effect For the type-b structure, the surface temperature of heater is reduced as the depth to width ratio is increased in any magnitude of heat load as shown in Fig. 3(a). The corresponding heat transfer coefficient is increased as the depth to width ratio is increased as shown in Fig. 3. As the heat load is increased, the surface temperature and heat transfer coefficient both are increased. The vapor quality in each kind of surfaces can be found in Fig. 3(c) which is proportional to the heat load. Basically, mass flow rate in this circulation system is increased as the heat load is increased as shown in Fig. 3(d) where the minichannels with larger depth to width ratio have larger amount of mass flow rate. The behavior of type-c is similar to that of type-b. The increase of depth to width ratio reduce the surface temperature and increase heat transfer coefficient in any magnitude of heat load as shown in Fig. 4. For any specific depth to width ratio, the surface temperature of type-c is lower than that of type-b as shown in Fig. 5. The better heat transfer performance of type-c results from its pin structure, which makes cooler liquid flow isotropically rather than in mono direction as of type-b, and it is therefore favorable to heat transfer. 952 Transactions of the Canadian Society for Mechanical Engineering, Vol. 40, No. 5, 2016

7 (a) (c) (d) Fig. 3. B-type boiling structure heat transfer performance in different depth to width ratio, gap = 0.3 mm. (a) Heater surface temperature, heat transfer coefficient, (c) vapor quality, (d) mass flow rate. (a) Fig. 4. C-type boiling structure heat transfer performance in different depth to width ratio, gap = 0.3 mm. (a) Heater surface temperature, heat transfer coefficient. Transactions of the Canadian Society for Mechanical Engineering, Vol. 40, No. 5,

8 Fig. 5. Surface temperature of all types of boiling structure in various heat loads. For all types of boiling structure, the vapor quality and the mass flow rate is increased as the heat load is increased. However, at higher heat load, e.g. 100W, the mass flow rate becomes insensitive to heat load because the increased vapor quality lead to larger frictional pressure drop. The nature of type-a which minichannels are perpendicular to main flow direction easily result in the dragging of fluidity and lower mass flow rate compared to those of type-b and type-c. This also results in most of type-a structures possess higher surface temperature of heater as shown in Fig Gap Effect The type B4 surface structure of evaporator with seven gap conditions from 0 to 3.56 mm are used to investigate the gap effect on the heat transfer performance. Figures 6(a) and 6 show the heat transfer performance is better as the gap is smaller than 0.51 mm. Basically, the surface temperature is reduced and heat transfer coefficient is increased as the gap is shrunk. However, there is optimal gap, 0.3 mm, shown in Fig. 6(c) which possesses lowest surface temperature and higher heat transfer coefficient, which is similar to the crossover gap in the report [1]. Figure 6(d) shows the channel without gap does not lead to flow blockage as the prediction. The bubble departure diameter of FC-72 is mm reported by Mudawar [1]. Hence, even in the zero gap condition, the vapor bubble can escape easily without blockage because of the sufficient large hydraulic diameter which cross section profile is 0.3 mm in width and 3 mm in depth. Nevertheless, if the gap is too large such as 1.27 mm or above, plenty bubbles rise up and congregate to be larger bubble which may result in blockage in the exit of evaporator as shown in Fig. 7(f). Therefore, there is optimal gap size, 0.3mm, which is favorable to the bubble departure and leaving out of the evaporator. The corresponding flow patterns in different gaps can be found in Fig Height of Condenser Effect Eight different height of condenser relative to evaporator are tested to study the influence on heat transfer. The results show higher altitude of condenser is beneficial to reduction of surface temperature of evaporator, but it does not affect proportionally on heat transfer coefficient as shown in Figs. 8(a) and 8. Figure 8(d) indicates the flow rate is larger once the condenser altitude is higher. The reason is that the higher condenser altitude results in larger pressure difference between riser and downcomer and leads to larger flow rate. At 954 Transactions of the Canadian Society for Mechanical Engineering, Vol. 40, No. 5, 2016

9 (a) (c) (d) Fig. 6. B4 type of boiling structure, heat transfer performance in different evaporator gaps. (a) Heater surface temperature, heat transfer coefficient, (c) optimal gap, (d) mass flow rate. The unit of gap is millimeter. (a) (c) (d) (e) (f) Fig. 7. Flow patterns in different evaporator gaps. (a) Gap 0, Gap 0.07, (c) Gap 0.13, (d) Gap 0.3, (e) Gap 0.51, (f) Gap 1.27 mm. Transactions of the Canadian Society for Mechanical Engineering, Vol. 40, No. 5,

10 (a) (c) (d) Fig. 8. C4 type of boiling structure, heat transfer performance in different condenser heights. (a) Heater surface temperature, heat transfer coefficient, (c) vapor quality, (d) mass flow rate. low heat loads, the flow rate is increased greatly as the heat load is increased. However, at higher heat loads, the flow rate is increased insensitively as the heat load is increased. The major reason is that higher heat load causes the increase of vapor quality which results in significant frictional pressure drop as shown in Figs. 8(c) and 8(d). For higher heat loads, the favorable effect of condenser height on heat transfer is augmented as shown in Figs. 8(a) and 8. For a condenser height, the flow rate is increased as the heat load is increased, if heat load is less than a limiting heat flux. Once heat load is larger than the limiting heat flux, the system becomes unstable [2]. The reason is that the increase of vapor quality induces more frictional pressure drop and leads to decreasing flow rate. This phenomenon is easily observed in lower condenser height, e. g. H 50. Especially, as the condenser height is lower than 40 cm, the insufficiency of pressure difference between riser and downcomer results in lower mass flow rate and larger vapor quality as shown in Figs. 8(c) and (d). The lower altitude of condenser results in the increase of vapor quality which induces larger frictional pressure loss and lower flow rate. This is in vicious spiral. However, at higher condenser height, the mass flow rate does not decrease as the heat load increases because the heat load is not large enough to reach its limiting heat flux. 956 Transactions of the Canadian Society for Mechanical Engineering, Vol. 40, No. 5, 2016

11 (a) (c) (d) Fig. 9. B4 type heat transfer performance in different evaporator inclined angles. (a) Heater surface temperature, heat transfer coefficient, (c) vapor quality, (d) mass flow rate Evaporator Inclination Effect The inclination of evaporator is varied in the range from 0 to 90 to study the bubble buoyancy effect on the heat transfer. This additional pressure drop across evaporator shown in Eq. (10) due to the gravitation by the inclination is necessary to be counted into Eq. (1). The results of Fig. 9(a) shows the inclination is not beneficial to dissipate heat from evaporator. Basically, the more inclination angle results in worse heat transfer as shown in Figs. 9(a) and 9. Among those, the worst case is at inclined angle at 60. Once the inclined angle is greater than 60 degrees, the fluidity is reduced because the gravitational pressure drop is augmented as shown in Eq. (10). At the same altitude of condenser, the available driving force for circulation is almost constant. Hence, the additional pressure drop due to the gravity will result in the reduction of mass flow rate. Consequently, the surface temperature is increased and heat transfer coefficient is reduced as shown in Figs. 9(a) and 9, respectively. vfg L1 2 g sin θ pg,1 2 = ln 1 + xout v f g xout vf Transactions of the Canadian Society for Mechanical Engineering, Vol. 40, No. 5, 2016 (9) 957

12 Regarding the additional pressure drop due to the inclination, a similar phenomenon had been reported by Aung and Li [8] whose numerical investigation pointed out the inclination indeed result in the significant pressure drop, reduction of mass flow rate and impairment of heat transfer coefficient. 6. CONCLUSIONS The depth to width ratio of enhanced boiling structure indeed plays a crucial role on the heat transfer performance of loop thermosyphon. Larger depth to width ratio is beneficial to the heat transfer. Among those types of enhanced boiling structure, the C-type possesses superior heat transfer performance to other types due to its isotropic flow property. The phenomenon of the optimal evaporator gap which possesses lowest surface temperature and higher heat transfer coefficient qualitatively agrees with the crossover gap in [1]. The condenser height indeed is good for reduction of surface temperature of heater due to larger pressure difference between riser and downcomer, if the height is not less than 50 centimeter. The inclination of evaporator on heat transfer may impair mass flow rate and heat transfer performance due to the caused additional pressure drop by gravity. REFERENCES 1. Mukherjee, S. and Mudawar, I., Smart pumpless loop for micro-channel electronic cooling using flat and enhanced surfaces, IEEE Transactions on Components and Packaging Technologies, Vol. 26, No. 1, pp , Garrity, P.T., Klausner, J.F. and Mei, R., Instability phenomena in a two-phase microchannel thermosyphon, International Journal of Heat and Mass Transfer, Vol. 52, pp , Kandlikar, S.G. and Balasubramanian, P., An experimental study on the effect of gravitational orientation on flow boiling of water in µm parallel minichannels, Journal of Heat Transfer, Vol. 127, pp , Zhang, H.Y., Flow boiling heat transfer in microchannel heat sinks of different flow orientations, in Proceedings of the ASME Summer Heat Transfer Conference, Singapore, pp , Wang, C.C., Chang, W.C., Dai, C.H., Lin, Y.T. and Yang, K.S., Effect of inclination on the convective boiling performance of a microchannel heat sink using HFE-7100, Experimental Thermal and Fluid Science, Vol. 36, pp , Collier, J.G., Convective Boiling and Condensation, 2nd ed., McGraw-Hill, Zhang, G., Liu, Z., Li, Y. and Gou, Y., Visualization study of boiling and condensation co-existing phase change heat transfer in a small and closed space with a boiling surface of enhanced structures, International Journal of Heat and Mass Transfer, Vol. 79, pp , Aung, N.Z. and Li, S., Numerical investigation on effect of riser diameter and inclination on system parameters in a two-phase closed loop thermosyphon solar water heater, Energy Conversion and Management, Vol. 75, pp , Transactions of the Canadian Society for Mechanical Engineering, Vol. 40, No. 5, 2016