On Experimental Study of Heat Transfer in Gravity Driven Water Film

Transcription

1 Proceedings of the rd IASME/WSEAS Int. Conf. on HEAT TRANSFER, THERMA ENGINEERING AND ENVIRONMENT, Corfu, Greece, August -, (pp-9) On Experimental Study of Heat Transfer in Gravity Driven Water Film SINKUNAS S., GYYS J., KIEA A. Department of Thermal and Nuclear Energy Kaunas University of Technology K.Donelaicio, T-9 Kaunas ITHUANIA Abstract: The object of this study as to thro more light on the heat transfer developments in the entrance region of a falling liquid film. The topic is also of importance because it aids the understanding and design of heat exchangers, turbines, refrigeration and air conditioning systems and etc. In many applications an estimation of the entrance region is often required. Velocity profiles predictions for the falling don film in the entrance region are given in the paper. Experimental investigation of heat transfer in the entrance region for the turbulent film as performed. The description of experimental set-up is presented. The research has been carried out ith ater film falling don a surface of vertical tube as Reynolds number ranged from 9. to.. The results of experiments are discussed ith the respect to the local heat transfer coefficient dependence upon Reynolds number and initial velocity of the film. Heat transfer ilization length as elished experimentally. Evaluation of heat losses in the test section is presented. Key-Words: Heat transfer, entrance region, turbulent film, vertical tube, ilization length Introduction Heat transfer has a very ide application from our everyday activities to various food processing equipments, to advanced nuclear and electronic processing facilities. It also has economical and environmental importance as sources of energy are valuable commodities and therefore various processes involving energy transport must to be optimized to meet future demands. Some of processes that involve the application of heat transfer are energy generation, separation technology, drying processes, storage and disposal of hazardous aste. Falling liquid films are often used in many technological processes, during hich heat and mass transfer exchange takes place. Gravity driven ater films are very frequent in thermal energy systems here evaporation processes occurs in boilers or nuclear reactors or vapor condenses in condensers []. They are also idely spread in chemical industry. In recent years, heat and mass transfer equipment operating ith the falling films on their external or internal surfaces have found a ide application in the food industry [-]. The intensity of evaporation, condensation, absorption, adsorption, burning and other technological processes depends upon ability to form and maintain a le smooth film of uniform thickness on the etted surface []. It is difficult to obtain the uniform liquid film distribution in the feed inlet. Specially designed liquid distributors at the feed inlet permit to explore more thin liquids films in the strict temperature regime. The most of the film apparatus for the thermal treatment of liquid products consists of vertical tubes ith falling films on their external surfaces [-]. In the case hen liquid film flos don a vertical tube the curvature of its surface and the film itself effects heat transfer characteristics on the surface and correspondingly thickness of the liquid film [-]. The cases that deal ith vertical plane film flo are investigated in [-]. The results of experimental research regarding the local heat transfer at liquid film flo on the horizontal tubes are presented in papers [7-9]. The questions as ho the hydromechanics parameters of gravitational turbulent liquid film are affected by initial velocity of the film have been investigated in []. Although many experimental methods has been used to determine film parameters on the etted surface, most investigations have been done in the ilized flo region. Fe orks considered the entrance region both theoretically and experimentally. In fact, the entrance region deals much more often ith the turbulent flo than ith the laminar flo.

2 Proceedings of the rd IASME/WSEAS Int. Conf. on HEAT TRANSFER, THERMA ENGINEERING AND ENVIRONMENT, Corfu, Greece, August -, (pp-9) Experimental Set-up The physical situation is considered here for a falling liquid film emerging from a ring-shape slot and floing don an external surface of vertical tube. otherise, the film accelerates and gets thinner hen initial average velocity of the film in the slot is less than an average velocity of ilized flo (case c). No doubt, something kind of the film thickness decrease is predictable even hen initial average velocity of the film is equal to an average velocity of ilized flo (case b). In order to generate the liquid film flo the experimental arrangement (Fig. ) as applied. d d > d a) b) d = from ater supply d < c) d Fig. Velocity profiles predictions for the falling don film in the entrance region at Re = const Fig. illustrates the assumed entrance region scheme. et us consider the entrance region model as Re = const. Reynolds number defines the character of the velocity profile in the slot. In real situation considered here the liquid film accelerates or decelerates to the limiting semi-parabolic velocity profile. The flo decelerates (case a) hen initial average velocity of the film exceeds an average velocity of ilized flo hile the thickness of the film increases until ilization takes place. And Fig. Schematic diagram of experimental set-up: - calorimeter; - liquid distributor; - slot distributive mechanism; - feed-pump; - exhaustpump; - inlet thermocouple; 7 - outlet thermocouple; 8 - centering bolts; 9 - gutter. - liquid reservoir The stainless steel tube mm in outside diameter ith the length of mm as employed in the experiment as a calorimeter. The fixing bolts at the end of tested tube alloed the possibility to regulate and to guarantee verticality of the tube. Water as pumped up to a liquid distributor by feed-pump. At the top end of the tube a slot distributive mechanism as installed to generate the uniform film flo. After floing don the test tube, the ater as gathered back to the reservoir. The gutter at the calorimeter end ensured a smooth falling of the ater into the reservoir. The surplus ater as discharged to the seerage by exhaustpump hile the fresh ater as supplied from ater

3 Proceedings of the rd IASME/WSEAS Int. Conf. on HEAT TRANSFER, THERMA ENGINEERING AND ENVIRONMENT, Corfu, Greece, August -, (pp-9) supply directly. Preliminary investigation has shon that the use of ater from plumbing did not influence on the demanded experiment accuracy. That is hy fresh ater as employed in the research. The temperature of falling don film as measured by to calibrated thermocouples. The location of a thermocouple in the liquid distributor ensured the measurement of film temperature at the inlet. The thermocouple installed at the end of calorimeter had determined a film temperature at the exit correspondingly. As heat flux along the tested section did not change, so to the demanded accuracy of the experiment it as assumed that the bulk mean temperature of the liquid film conforms linear regularity. That circumstance alloed determining the liquid film temperature at any cross-section of the tested section accurately. For the heat transfer research of falling don ater film on the surface of vertical tubes the electric circuit (Fig. ) as applied. The electric current supplied for the calorimeter provided a steady heat flux on the experimental section. In order to convert an alternating current into a direct current the rectifier as used. Voltages drop on the knon value resistor fitted in as shunt determined the electric current strength in the circuit. Its readings ere taken from the millivoltmeter. Voltage value on the calorimeter as measured by voltmeter. To investigate the heat transfer intensity to the liquid film falling don a vertical surface the calorimeter (Fig. ) as constructed. Fig. Calorimeter: - thin-alled stainless steel tube; - tip; - elastic tube; - thermocouples; - elastic plastic ring; - head of thermocouple The stainless steel tube mm in outside diameter and mm length as employed in the experiment as the test section. The temperature of inner tube all as determined by. -. mm copper-constantan thermocouples fixed by pressing them to the inner all surface ith elastic plastic rings. In order to avoid the electric current influence on the thermocouples their heads ere covered ith a thin layer of dielectric lacquer. Thirty thermocouples ere located along the inner surface of the calorimeter, by three of them in each of ten cross sections respectively. The millivoltmeter registered the values of thermocouples electromotive force. Fig. Electric circuit: - calorimeter; - voltage regulator; - rectifier; - millivolmeter; - shunt; - voltmeter ocal Heat Transfer for a Turbulent Film Flo on a Vertical surface The ater film floing don a surface of vertical tube as used in experiments. The experiments ere provided for Reynolds number ranged from 9. to.. The temperature of the calorimeter surface and the film, electric current, voltage ere measured and recorded during the experiment. After registration of electric current and voltage the heat flux density on the calorimeter surface as

4 Proceedings of the rd IASME/WSEAS Int. Conf. on HEAT TRANSFER, THERMA ENGINEERING AND ENVIRONMENT, Corfu, Greece, August -, (pp-9) calculated. When records of heated tube surface and film flo temperatures ere performed, the difference of temperature T (beteen the mean temperatures of film T f and tube surface T ) as calculated. ocal heat transfer coefficient as computed by formula α = q T () α -,W/(m K) x, m Fig. Variation of local heat transfer coefficient in the entrance region of the film flo don a vertical surface: - Re=9, ε=.89; - 9, ε=.; -, ε=. Experimental data are presented in Fig.. As e can see, alteration of the local heat transfer coefficient in the entrance region is complicated. Three different regions may be delineated along the length of film flo in a case hen initial average velocity of the film in the liquid distributor is less than an average velocity of ilized flo. The significant decrease of the local heat transfer coefficient at some distance from liquid distributor hile reaching minimal value is seen at the first region. This phenomenon one can explain by the development of thermal boundary layer and its laminar nature. In the second region, fluid fluctuations begin to develop hile heat transfer increases to a maximum value. The beginning of heat transfer ilization takes place in the third region of the film flo. Augmentation of a thermal boundary layer terminates ith the film thickness. The variation of local heat transfer in the entrance region hen initial average velocity of the film exceeds or is equal to an average velocity of ilized film is not high. As e can see from Fig., in all cases the heat transfer ilization takes place at the distance.7 m from the liquid distributor hen Reynolds number ranged from 9. to.. Evaluation of the Heat osses in the Test Section The heat losses in the experimental section must be evaluated. Therefore, the method for heat losses evaluation as proposed. Assume that the tube (Fig. ) is heated electrically. The heat balance equation for elementary volume of the tube δ can be ritten as follos dϑ d dϑ qδ λδ = αϑ λδ ϑ + () here: ϑ = T T et us denote that α = a λδ and f q = b () λδ Rearranging Eq. () ith the folloing boundary conditions dϑ ϑ = ϑ, = for x = and ϑ = ϑ for x = () e obtain the folloing equation d ϑ aϑ +.b = Solution of the Eq. () leads to the expression [ ab( ϑ ϑ ) a ( ϑ ϑ )] = exp ( a ) aϑ b. + aϑ b = () () If notation ϑ = kb a is applied, the Eq. () can be reritten to the folloing form ( k) exp( a ) ( ) + exp a b ϑ = (7) a b = a It is not difficult to make sure that Q Fα (8) Considering that ϑ = q α, the real heat flux in the experimental section can be determined by the folloing expression

5 Proceedings of the rd IASME/WSEAS Int. Conf. on HEAT TRANSFER, THERMA ENGINEERING AND ENVIRONMENT, Corfu, Greece, August -, (pp-9) ( k) exp( a ) ( ) + exp a Q q = (9) F It should be noted that the largest heat losses exist hen ϑ = and k = hile they are not at k =. The folloing inequality has taken place in our research for the experimental section in all cases ( k) exp( a ) + exp( a ).99 () Therefore, the heat losses during the experiments ere minimal. Fig. Experimental tube Conclusions Heat transfer characteristics in the entrance region of the turbulent film flo ere studied experimentally. All experimental tests ere conducted on the outside surface of vertical tubes. On the basis of experimental results the folloing conclusions have been dran.. The turbulent film flo is very complicated and difficult for theoretical study. It is ell knon that small disturbances associated ith distortion in the fluid streamlines of laminar film can eventually lead to turbulent conditions. For a larger Reynolds number the inertia forces are sufficiently large to amplify the disturbances and a transition to turbulence occurs.. The existence of the turbulent flo can be advantageous in the sense of providing increased heat transfer rates. Hoever, the motion is extremely complicated and difficult to study analytically. In this case the only possible ay is the experiment.. The determining effect of film generation on heat transfer takes place in the entrance region of the film flo don a surface of vertical tube. The x d δ experimental data revealed that Reynolds number and initial velocity has a significant influence on local heat transfer. It is obtained that heat transfer ilization occurs at.7 m distance from the liquid distributor hen Re > 9... Method evaluating the heat transfer losses in the experimental section has been proposed. It as found that the value of exponent have appeared as k.99 during the experiments therefore one can confirm that the heat loss is negligible. Nomenclature d inner diameter of the tube (m) F surface area of the tube (m ) length of heated tube (m) Q heat flux (W) q heat flux density (W/m ) Re Reynolds number of liquid film, [ Г/( ρ v)] T temperature (K) average film velocity (m/s) x longitudinal coordinate (m) Greek etters α heat transfer coefficient [W/(m K)] Г etted density [kg/(ms)] δ thickness of tube all (m) ε relative film velocity, d λ thermal conductivity [W/(m K)] ν kinematic viscosity (m /s) ρ liquid density (kg/m ) Subscripts d distributor f film flo ilized flo all of tube References: [] Gantchev, B.G., Cooling of Nuclear Reactor Elements By Falling Film Flo, Energoatomizdat: Moskva, 987. [] Bylund, G., Dairy Processing Handbook, Tetra Pak Processing Systems AB: und, 99. [] eis, M.J., Young, T.W., Breing, Chapman & Hall: ondon, 998. [] Martzeniuk, A.C., and Stabnikov, V.U., Film Heat-Mass Transfer Apparatus in a Food Processing Industry, ight and Food Processing Industry: Mosco, 99.

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