HEAT TRANSFER STUDIES IN A VERTICAL TUBE OF CLOSED-LOOP THERMOSIPHON

Abstract

Closed loop thermosiphons provide an efficient and economical means of heat transfer in diverse fields of application. The rate of heat transfer and thermally induced flow in these systems interact with each other and depend upon a number of design and operating para meters in a very complex manner. The prediction of these rates is the main requirement for the design of units using a closed-loop thermosiphon system. The study of natural convective flow and heat transfer in confined spaces has been conducted by many workers in the past. These studies are mostly on vertical channels form ing open thermosiphons. The fluids used were invariably air or gas with Prandtl number around 0.7. Very few studies on heat transfer in single phase natural convective flow of liquids in closed-loop circulation systems seems to have been reported. The hydrodynamics and heat transfer in thermosiphon roboilers have been studied by a number of investigators over the past two decades. A critical review of the published IV work reveals that most of the studies have been conducted for uniform wall temperature heating conditions with saturated liquids. Some studies at high heat fluxes are also confined to saturated boiling and very limited range of other important parameters. No systematic study seems to be available in the literature which gives the exclusive effect of inlet degree of subcooling and submergence on the performance of thermosiphon reboilcrs. In the present work, a systematic experimental study of heat transfer in the vertical tube of closed-loop thermosiphons, both single as well as two-phase systems, has been carried out under low values of uniform wall flux heating conditions. The experimental apparatus used for the study on single phase systems consisted of a vertical copper tube as test section connected to a large capacity vessel and a jacketted pipe forming a closed-loop circulation system. The test section was of 19.05 mm inside diameter. 1.58 mm wall thick ness and 940 mm heated length. The electrical heating elementnichrome wire, in porcelain beads, v/as uniformly wound over the tube. A stabilized A.C. electrical input to the heater was regulated through autotransformers and measured by calibrated wattmeters. The tube wall temperatures were measured by ten copper-constantan thermocouples whose beads were embedded in the wall thickness at locations 100 mm apart starting from the lower end of the heated length. The mixed mean liquid temperatures were measured by moans of copperconstantan thermocouple probes inserted at the two ends of the heated tube. Three liquids-water, ethylene glycol and glycerol were used as test liquids for single phase natural circulation studies. Experimental data were generated imposing constant heat flux ranging from 1.77x10* to 1.066xKr W/m . As a consequence of changing inlet temperature to the test section and heat flux, wide range of variation in physical properties was observed in terms of pertinent dimensionless groups. The values of Gr.Pr ranged from 9.07x10 to 2.11x10', and the corresponding values of Re due to induced flow from O.39 to 445. The test section of single vertical tube thermo siphon reboiler consisted of an electrically heated stain less steel tube of 21.44 mm inside diameter. 2.O3 mm wall thickness and 1440 mm heated length. The uniform wall heat flux was obtained from the measured electrical input. In order to get the local heat transfer coefficients, the tube wall temperatures were measured by sixteen copper-constantan thermocouples whose beads were spot welded on the tube surface at locations 100 mm apart starting from the lower end. The inlet and exit liquid temperatures were measured by means of copper-constantan thermocouple probes separatel}'. The upper end of the test section was connected to a vapour-liquid separator, the vapour line from which led to a total condenser. The liquid line from the separator and condensate line from condenser wer©. connected to a jacketted vertical tube which vi served as cold leg. The lower ends of test section and cold leg were joined together through a horizontal tube forming a closed-loop circulation system. The experiments were conducted with five test liquids-acetone, ethyl acetate, propan-2-ol, water and toluene having wide range of boiling points and thermophysical properties. The uniform heat fluxes in the range of 3.OXIO-5 to 2*4x10 W/ra were used. The degrees of subcooling at the test section inlet were varied over a wide range for all test liquids. During the experimentation submergence levels around 100,80 and 50 percent were maintained. The experimental data of single phase systems were used to obtain wall and liquid temperature distributions along the tube iength at various levels of heat flux and inlet liquid temperatures for all the three test liquids investigated. The heat transfer coefficients were computed from imposed heat flux and the resulting local values of temperature difference and their variation along the tube length have been discussed. The values of heat transfer coefficient have been found to increase with heat flux due to the higher levels of tempera ture setup and increased induced flow rates of the liquids. At the same heat flux, the transfer coefficients show a decreas ing order for water, ethylene glycol and glycerol. As a result of data analyses, dimensionlcss correlations to predict local and average heat transfer coefficients have been developed. vii For local heat transfer coefficients, °*388 n^ 0*363 For average heat transfer coefficients, (/N„ un;ayg . 0.049(,Gr.Pr)a0v.5g84 (Re;a-0vg.131(//d)-0.622 In natural circulation boiling of liquids, the experimentally obtained wall and liquid temperature distri butions along the tube length revealed the varying lengths of effective boiling and non-boiling sections. In between, the point of onset of boiling for all the test liquids under various operating conditions were identified and discussed. The non-boiling region was identified by the linearly vary ing' liquid and wall temperatures along trie tube length under the conditions of constant heat flux. The zone of transition from nonboiling through subcooled boiling to fully developed saturated boiling was marked by the attainment .of maximum wall temperature followed by e sudden decrease in its value and subsequently becoming aOfisost constant* The heat transfer coefficient remained almost invariant with length over the non-boiling section of the tube. It jumped to much higher values at the onset of boiling condi tions and became constant in fully developed boiling region of the tube length. The variation of h with z was found to be similar for all systems and was affected by heat flux, inlet liquid subccoling and submergence. Vlll The onset of fully developed boiling required a minimum degree of wall superheat for a given liquid and heat transfer surface. Based on the analytical study of Yin and Abdelmessih[5?o] the following expression for wall superheat as a function of q has been developed 2 20* T The constants a and b in the equation are as under; System a Acetone 24.35 Ethyl acetate 23.40 Propan-2-ol 19.89 Vaster I6.42 Toluene 15.84 0.296xl<T3 0.83lxl0~3 0.477xlO~3 0.106x10*"^ 0.343x10*"^ The distance along the heated tube required for the onset of fully developed boiling of a liquid depends upon wall heat flux, inlet liquid subcooling and submergence. An empirical correlation for predicting this length has been developed in the following forms r 2 OB n, jt-x 100J =<VPsb> (}\ nc n. ub' The values of constant C^ and exponents n,, n~ and B> for all the five test liquids have been evaluated and are as under 5 ix Acetone 0.095 -0.2890 0.260 1.450 Ethyl acetate 0.266 -CI63O O.34O O.995 Propan-2-ol 0.5ILxlO"~3 O.443O 0.068 1.993 Water 0.54x10":'3 0.0014 O.I33 2.405 Toluene 1«55x10"3 0.4370 0.120 1.740 The heat transfer coefficient in fully developed boiling region has been found to be strongly dependent on heat flux due to the dominant role of nucleate boiling mechanism in the experimental conditions of present investigation. The effect of inlet liquid subcooling and submergence on the magnitude of hB was observed to be negligible. However, the length of tube over which fully developed boiling occurred was found to increase v/ith the decrease in the values of inlet liquid subcooling and submergence. Thus, the boiling heat transfer coefficient averaged out over the fully developed boiling section of the tube has been expressed as a function of q in the following power lav; relationship. hB = c qn The values of c and n are as underi System c n Acetone 45.31 0,387 Ethyl acetate 75.18 O.332 Propan-2-ol 54.36 0.351 Water 17.51 0#556 Toluene 37.29 O.38I IX • System % %L & !2§ Acetone 0.095 -0.2890 0.260 1.450 Ethyl acetate 0.266 -0,1630 O.34O 0.995 Propan-2-ol 0. 51x10"•3 O.443O 0.068 1.993 Water 0.54x10*:•3 0.0014 O.I33 2.405 Toluene 1.55x10"-3 0.4370 0.120 1.740 The heat transfer coefficient in fully developed boiling region has been found to be strongly dependent on heat flux due to the dominant role of nucleate boiling mechanism in the experimental conditions of present investigation. The effect of inlet liquid subcooling and submergence on the magnitude of hB was observed to be negligible. However, the length of tube over which fully developed boiling occurred was found to increase v/ith the decrease in the values of inlet liquid subcooling and submergence. Thus, the boiling heat transfer coefficient averaged out over the fully developed boiling section of the tube has been expressed as a function of q in the following power law relationships hB = c qn The values of c and n are as unders System c n Acetone 45.31 0,387 Ethyl acetate 75.18 O.332 Propan-2-ol 54.36 0.351 Water ±7,51 0*556 Toluene 37.29 O.38I The variation of hB with q also showed a good agreement with the experimental data from earlier studies of similar nature. The rate of" heat transfer in nonboiling region of the reboilcr- tube vras found to be mainly governed by natural convective flow of liquids in the similar manner as in the case of single phase thermosiphon. However, the net flow of liquid through the reboiler tube was higher than that in the single phase thermosiphon duo to the liquid vapourization in the upper section. The average values of heat transfer coefficient for all the five test liquids were best correlated in terms of same dimensionless groups, used in single phase thermosiphon, by the following equation with a maximum deviation of + 30 per cent NuNB 0.481 n nu zr,u -G#23 -0.0126(Gr.Pr) ite££*(3&) ' A correlation for boiling heat transfer coefficient has also been developed in terms of suitable dimensionless groups. 0.462 , 0,0134 1,014 VHC0 1.743 NuB =6.l42(PeB) (^) &r) (-£-) y"tt VL The data points of all the five liquids were found to be well within + 30 per cent deviation from the equation. Although, the correlation has been developed based on experimental data at very low values of heat flux and XI small vapour fractions, its form is fairly general and may be extended to other ranges of parameters. None of the earlier correlations, available in literature, has been found to agree weH-ftlth the experimental data of present investigation.