Boiling heat transfer and pressure drop of ammonia-lithium nitrate solution in a plate generator

This paper presents the thermal and pressure-drop experimental evaluation of a fusion plate heat exchanger (PHE) during boiling conditions of a solution of lithium nitrate in ammonia. The data are representative of the working conditions of generators in single-effect absorption chillers. The solution flow rate and outlet temperature were modified in the ranges of 0.041–0.083 kg/s and 78–95 °C, respectively. Correlations for single-phase-flow heat transfer are used to characterize the boiling heat transfer. The influences of the heat flux, mass flux and exit-vapour quality are analyzed. Boiling heat-transfer coefficients and correlations for the Nusselt number are obtained. Results are compared with Cooper’s and Ayub’s correlations for boiling heat transfer. Pressure drop in the solution side was also measured and one correlation was obtained to characterize the frictional pressure drop under boiling conditions.     

Recirculation model of kettle reboiler

The present paper deals with the simulation of a kettle reboiler. Considering rectangular tube sheet, concept of internal recirculation developed in a kettle reboiler during boiling, changes in physico-thermal property of liquid and liquid vapour mixture with temperature and pressure and using empirical correlations, a hydrodynamic model has been developed to determine pressure drop, vapour quality, recirculation rate, boiling regime, and heat transfer coefficient at various tube rows of the bundle.Results show, recirculation rate in a reboiler has been found to vary with the heat flux and pressure. Further, at a given value of heat flux and pressure vapour quality, mass flux, and heat transfer coefficient have been found to increase gradually from bottom to top tube row of the bundle.     

Examination of heat transfer correlations and a model for flow boiling of R134a in small diameter tubes

Analysis of various existing correlations including a three-zone evaporation model is made using a comparison with recent experimental results obtained in this study. Flow boiling heat transfer experiments were conducted with two stainless steel tubes of internal diameter 4.26 mm and 2.01 mm. The working fluid was R134a and parameters were varied in the range: mass flux 100–500 kg/m²s; pressure 8–12 bar; quality up to 0.9; heat flux 13–150 kW/m². The local heat transfer coefficient was independent of vapour quality when this was less than about 40–50% in the 4.26 mm tube and 20–30% in the 2.01 mm tube. Local transient dryout was deduced when the quality was above these values. Furthermore, at high heat flux values the heat transfer coefficient decreased with vapour quality for the entire quality range indicating early occurrence of dryout.Existing correlations, which are based on large tube boiling processes, do not predict the present small diameter data to a satisfactory degree. A better agreement is observed with the recent, state-of-the-art, three-zone evaporation model. However, the model does not predict the effect of diameter and the partial dryout. Nevertheless, the observation suggests that the flow pattern based modelling approach performs at least as well as empirical correlations that are based on macroscale modelling. Aspects of the model that need further consideration are also proposed in this study.     

Boiling heat transfer in narrow channels with offset strip fins: Application to electronic chipsets cooling

An experimental study on saturated flow boiling heat transfer of HFE-7100 in vertical rectangular channels with offset strip fins is presented. The experiments have been carried out at atmospheric pressure, over a wide range of vapour quality and heat fluxes up to 1.8 × 10⁵ W/m². The local boiling heat transfer coefficient has been obtained from experiments and analysed by means of Chen superposition method. Some correlations for convective boiling and nucleate boiling heat transfer coefficients have been considered. A good agreement has been found with Feldman et al. [3] correlation for convective boiling heat transfer and Kim and Sohn [8] correlations for nucleate boiling heat transfer.A closed circuit for electronic chipsets cooling, with the same evaporator as that studied in the first part of the paper, has been studied. Thermal performances of this system have been measured and compared with those of a circuit with the same components but no internal fins in the evaporator. The results have shown that for high heat loads the inner geometry of evaporator does not influence the two-phase heat transfer. For low heat loads, offset strip fins evaporator gives better performances than no fins evaporator.     

Experimental investigation of flow boiling heat transfer of jet impingement on smooth and micro structured surfaces

Flow boiling heat transfer experiments using R134a were carried out for jet impingement on smooth and enhanced surfaces. The enhanced surfaces were circular micro pin fins, hydrofoil micro pin fins, and square micro pin fins. The effects of saturation pressure, heat flux, Reynolds number, pin fin geometry, pin fin array configuration, and surface aging on flow boiling heat transfer characteristics were investigated. Flow boiling experiments were carried out for two different saturation pressures, 820 kPa and 1090 kPa. Four jet exit velocities ranging from 1.1–4.05 m/s were investigated. Flow boiling jet impingement on smooth surfaces was characterized by large temperature overshoots, exhibiting boiling hysteresis. Flow boiling jet impingement on micro pin fins displayed large heat transfer coefficients. Heat transfer coefficients as high as 150,000 W/m² K were observed at a relatively low velocity of 2.2 m/s with the large (D = 125 μm) circular micro pin fins. Jet velocity, surface aging, and saturation pressure were found to have significant effects on the two-phase heat transfer characteristics. Subcooled nucleate boiling was found to be the dominant heat transfer mechanism.     

Comparison of Six Typical Correlations for Upward Flow Boiling Heat Transfer with Kerosene in a Vertical Smooth Tube

The present article is aimed at evaluating six typical flow boiling heat transfer correlations selected from the open literature with experimental results. The selected correlations are correlations of Chen, Shah, Gungor and Winterton, Liu and Winterton, Klimenko, and Kandlikar. Experiments of upward flow boiling heat transfer with kerosene in a vertical smooth tube were conducted. The test tube has a length of 2.5 m and its outer and inner diameters are 19 mm and 15 mm, respectively. The experiments were performed at an absolute atmospheric pressure of 3. The input heat flux ranged from 28.5 to 93.75 kW/m² and the mass fluxes were selected at 410, 610, and 810 kg/m² s, respectively. The experimental flow boiling heat transfer coefficients were compared with flow boiling heat transfer coefficients calculated with the six typical correlations. By comparison, the most suitable correlations are recommended for the calculation of flow boiling heat transfer coefficients with kerosene in a smooth tube.     

Effect of pressure, subcooling, and dissolved gas on pool boiling heat transfer from microporous, square pin-finned surfaces in FC-72

The present research is an experimental study of the effects of pressure, subcooling, and non-condensable gas (air) on the pool nucleate boiling heat transfer performance of microporous enhanced finned surfaces. The test surfaces, solid copper blocks with 1-cm² bases and 5×5 square pin-fin arrays of 2, 4 and 8 mm fin lengths, were immersed in FC-72. The test conditions included an absolute pressure range of 30-150 kPa and a subcooling range of 0 (saturation) to 50 K. Effects of these parameters on nucleate boiling and critical heat flux (CHF) were investigated. In addition, differences between pure subcooled and gas-saturated conditions as well as horizontal and vertical base orientations were also investigated. Results showed that, in general, the effects of pressure and subcooling on both nucleate boiling and CHF were consistent with previously tested flat surface results, however, subcooling was found to significantly affect the high heat flux region of the microporous finned surfaces nucleate boiling curves. The relative enhancement of CHF from increased subcooling was greater for the microporous surface than the plain surface but less than a microporous flat surface. The horizontal orientation (horizontal base/vertical fins) was found to be slightly better than the vertical orientation (vertical base/horizontal fins). Correlations for both nucleate boiling and CHF for the microporous surfaces were also developed.     

Flow boiling of liquid nitrogen in micro-tubes: Part II – Heat transfer characteristics and critical heat flux

This paper is the second portion of a two-part study concerning the flow boiling of liquid nitrogen in the micro-tubes with the diameters of 0.531, 0.834, 1.042 and 1.931 mm. The contents include the heat transfer characteristics and critical heat flux (CHF). The local wall temperatures are measured, from which the local heat transfer coefficients are determined. The influences of heat flux, mass flux, pressure and tube diameter on the flow boiling heat transfer coefficients are investigated systematically. Two regions with different heat transfer mechanism can be classified: the nucleate boiling dominated region for low mass quality and the convection evaporation dominated region for high mass quality. For none of the existed correlations can predict the experimental data, a new correlation expressed by Co, Bo, We, K_p and X is proposed. The new correlation yields good fitting for 455 experimental data of 0.531, 0.834 and 1.042 mm micro-tubes with a mean absolute error (MAE) of 13.7%. For 1.931 mm tube, the flow boiling heat transfer characteristics are similar to those of macro-channels, and the heat transfer coefficient can be estimated by Chen correlation. Critical heat flux (CHF) is also measured for the four tubes. Both the CHF and the critical mass quality (CMQ) are higher than those for conventional channels. According to the relationship that CMQ decreases with the mass flux, the mechanism of CHF in micro-tubes is postulated to be the dryout or tear of the thin liquid film near the inner wall. It is found that CHF increases gradually with the decrease of tube diameter.     

Flow pattern and heat transfer characteristics of R-134a refrigerant during flow boiling in a horizontal circular mini-channel

Flow boiling heat transfer of R-134a refrigerant in a circular mini-channel, 600 mm long with a diameter of 1.75 mm, is investigated experimentally in this study. The test section is a stainless steel tube placed horizontally. Flow pattern and heat transfer coefficient data are obtained for a mass flux range of 200–1000 kg/m² s, a heat flux range of 1–83 kW/m² and saturation pressures of 8, 10, and 13 bar. Five different flow patterns including slug flow, throat-annular flow, churn flow, annular flow and annular-rivulet flow are observed and the heat transfer coefficient data for different flow patterns are presented. The heat transfer coefficient increases with increasing heat flux but is mostly independent of mass flux and vapour quality. In addition, it is indicated from the experiments that the higher the saturation pressure, the lower is the heat transfer coefficient. Comparisons of the present data with the existing correlations are also presented.     

Condensing heat transfer and pressure drop characteristics of hydrocarbon refrigerants

This paper presents the experimental results of condensing heat transfer coefficients and pressure gradients of HC refrigerants (e.g. R-1270, R-290 and R-600a) and R-22 in horizontal double pipe heat exchangers, having two different internal diameters of 12.70 mm and 9.52 mm (OD), respectively. Both the local condensing heat transfer coefficients and pressure drops (inside the tube) of hydrocarbon refrigerants were higher than R-22. The average condensing heat transfer coefficient increased with the mass flux. The experimental heat transfer coefficients agreed with the correlations of Shah, Travis and Cavallini–Zecchin’s to within ±20%. These results can be useful in the design of new heat exchangers involving hydrocarbon refrigerants for future air-conditioning systems.     

Flow boiling heat transfer coefficient of R-134a/R-290/R-600a mixture in smooth horizontal tubes using varied heat flux method

An experimental study on in-tube flow boiling heat transfer of R-134a/R-290/R-600a refrigerant mixture has been carried out under varied heat flux test conditions. The heat transfer coefficients are experimentally measured at temperatures between ?8 and 5 °C for mass flow rates of 3–5 g s^?1. Acetone is used as a hot fluid which flows in the outer tube of diameter 28.57 mm while the refrigerant mixture flows in the inner tube of diameters 9.52 and 12.7 mm. By regulating the acetone flow conditions, the heat flux is maintained between 2 and 8 kW/m² and the pressure of the refrigerant is maintained between 3.2 and 5 bar. The comparison of experimental results with the familiar correlations shows that the correlations over predict the heat transfer coefficients for this mixture when stratified and stratified-wavy flow prevail. Multiple regression technique is used to evolve and modify existing correlations to predict the heat transfer coefficient of the refrigerant mixture. It is found that the modified version of Lavin–Young correlation (1965) predicts the heat transfer coefficient of the considered mixture within an average deviation of ±20.5 %.     

Boiling on a Tube Bundle: Part II—Heat Transfer and Pressure Drop

An extensive experimental study was undertaken to measure nucleate pool boiling heat transfer coefficients, local bundle boiling heat transfer coefficients, and two-phase bundle pressure drops for R134a and R236fa on one plain tube bundle configuration. The experimental database allowed the refinement of frictional pressure drop models previously developed at the Laboratory of Heat and Mass Transfer. Together with the new onset of dryout prediction method presented in part I (preceding article in this issue), this constitutes a significant improvement in such prediction methods. The local bundle boiling heat transfer data highlighted the dependency of the heat transfer coefficient on the heat flux as expected for the present conditions. The new method was proposed and worked well versus the present database and was also validated against additional refrigerants from independent studies. It was proven to also work reasonably well for falling film evaporation data, proving the new prediction method is applicable for a wide range of operating conditions.     

Experiments on the characteristics of evaporation of R410A in brazed plate heat exchangers with different geometric configurations

Experiments on the evaporative heat transfer and pressure drop in the brazed plate heat exchangers were performed with refrigerants R410A and R22. The plate heat exchangers with different 45°, 35°, and 20° chevron angles are used. Varying the mass flux of refrigerant (13–34 kg/m² s), the evaporating temperature (5, 10 and 15 °C), the vapor quality (0.9–0.15) and heat flux (2.5, 5.5 and 8.5 kW/m²), the evaporation heat transfer coefficients and pressure drops were measured. The heat transfer coefficient increases with increasing vapor quality and decreasing evaporating temperature at a given mass flux in all plate heat exchangers. The pressure drop increases with increasing mass flux and quality and with decreasing evaporating temperature and chevron angle. It is found that the heat transfer coefficients of R410A are larger than those of R22 and the pressure drops of R410A are less than those of R22. The empirical correlations of Nusselt number and friction factor are suggested for the tested PHEs. The deviations between correlations and experimental data are within ±25% for Nusselt number and ±15% for friction factor.     

Condensation heat-transfer and pressure drop characteristics of refrigerant R-290/R-600a–oil mixtures in serpentine small-diameter U-tubes

Flow condensing experiments for refrigerant R-290, and R-600a mixed with the lubricating oil (EMKARATE RL 32H) in serpentine small-diameter (2.46 mm) U-tubes are reported. The tests were run at the saturation temperature of 40 °C, vapor qualities of 0.41–0.82, mass flux of 300–600 (kg/m²s) and inlet oil concentrations from 0 to 5 mass% oil. It was found that the condensation heat-transfer coefficients increased as mass flux values, vapor quality and the number of tube bends increased, but it decreased as the oil concentration increased. In addition, the two-phase pressure drops increased with increases in mass flux values, the number of tube bends and the oil concentration.     

Investigation of flow phenomena in a kettle reboiler

Two-phase flow phenomena were investigated while boiling R113 and n-pentane in a 241-tube thin slice kettle reboiler. For heat fluxes between 10 and 40 kW/m², row pressure drop measurements were made in three columns and visual observations of the flow patterns were recorded by a video camera. The height of the two-phase mixture above the tube bundle was also varied. The results revealed that the height of the mixture had little effect on the row pressure drop distribution in each column. At heat fluxes below 10 kW/m², the pressure drops were reasonably constant. However, at heat fluxes greater than this, the row pressure drop continuously declined.Two one-point-five-dimensional models were developed, one to aid the investigation of static liquid driven lateral flow in the tube bundle, and another to aid the investigation of the cause of the change from reasonably constant to continually declining row pressure drop. The data and the analysis showed that the flow within the tube bundle was always two-dimensional and that the flow pattern was dominated by the static liquid at the tube bundle edge when the heat flux was less than 10 kW/m². This corresponded to the bubbly flow regime. At larger heat fluxes, the flow pattern changed to intermittent flow. The change occurred when the Kutateladze number was 1.09. Declining row pressure drops occurred in this latter flow regime.     

High heat flux flow boiling in silicon multi-microchannels – Part I: Heat transfer characteristics of refrigerant R236fa

This article is the first in a three part study on flow boiling of refrigerants R236fa and R245fa in a silicon multi-microchannel heat sink. The heat sink was composed of 67 parallel channels, which are 223 μm wide, 680 μm high and 20 mm long with 80 μm thick fins separating the channels. The base heat flux was varied from 3.6 to 221 W/cm², the mass velocity from 281 to 1501 kg/m² s and the exit vapour quality from 2% to 75%. The working pressure and saturation temperature were set nominally at 273 kPa and 25 °C, respectively. The present database includes 1217 local heat transfer coefficient measurements, for which three different heat transfer trends were identified, but in most cases the heat transfer coefficient increased with heat flux and was almost independent of vapour quality and mass velocity. Importantly, it was found for apparently the first time that the heat transfer coefficient as a function of vapour quality reaches a maximum at very high heat fluxes and then decreases with further increase of heat flux.     

Saturated flow boiling heat transfer and pressure drop of refrigerant R-410A in a vertical plate heat exchanger

Saturated flow boiling heat transfer and the associated frictional pressure drop of the ozone friendly refrigerant R-410A (a mixture of 50 wt% R-32 and 50 wt% R-125) flowing in a vertical plate heat exchanger (PHE) are investigated experimentally in the study. In the experiment two vertical counter flow channels are formed in the exchanger by three plates of commercial geometry with a corrugated sinusoidal shape of a chevron angle of 60°. Upflow boiling of saturated refrigerant R-410A in one channel receives heat from the downflow of hot water in the other channel. The experimental parameters in this study include the refrigerant R-410A mass flux ranging from 50 to 125 kg/m² s and imposed heat flux from 5 to 35 kW/m² for the system pressure fixed at 1.08, 1.25 and 1.44 MPa, which respectively correspond to the saturated temperatures of 10, 15 and 20 °C. The measured data showed that both the boiling heat transfer coefficient and frictional pressure drop increase almost linearly with the imposed heat flux. Furthermore, the refrigerant mass flux exhibits significant effect on the saturated flow boiling heat transfer coefficient only at higher imposed heat flux. For a rise of the refrigerant pressure from 1.08 to 1.44 MPa, the frictional pressure drops are found to be lower to a noticeable degree. However, the refrigerant pressure has very slight influences on the saturated flow boiling heat transfer coefficient. Finally, empirical correlations are proposed to correlate the present data for the saturated boiling heat transfer coefficients and friction factor in terms of the Boiling number and equivalent Reynolds number.     

Heat transfer characteristics of two-phase He I (4.2 K) thermosiphon flow

In the framework of the cryogenic cooling system design of a large superconducting magnet under construction at CERN-Geneva, heat transfer in two-phase He I natural circulation loop has been investigated experimentally. The experiments were conducted on a 2 m thermosiphon loop with copper tube of 10 mm inner diameter uniformly heated over a length of 0.95 m. All data were obtained near atmospheric pressure. Evolution of the exit vapour quality and wall superheat as a function of heat flux are presented and analyzed. A comparison between the two-phase heat transfer coefficient h_TP determined in our study and the most relevant correlations available in literature is made. Further, we predict h_TP with a correlation based on the combining effects of forced convection and nucleate boiling by a power-type asymptotic model. Finally, we present the boiling crisis study and we propose a critical heat flux correlation as a function of channel height to diameter ratio (z/D) to model our experimental results.     

Evaporation heat transfer and pressure drop characteristics of R-290 (propane), R-600 (butane), and a mixture of R-290/R-600 in the three-lines serpentine small-tube bank

This paper reports an experimental investigation of heat-transfer and pressure-drop behavior of R-290, R-600, and R-290/R-600 in the three-lines serpentine small-diameter (2.46 mm) tube bank. Heat transfer coefficients and pressure drop characteristics are measured for a range of heat flux (5–21 kW/m²), mass flux (250–500 kg/m² s), equilibrium mass quality (0–0.86), and the fixed mixture composition ((R-290/R-600 (55 wt.%/45 wt.%)). The results show that the flow boiling heat transfer coefficients for R-290, R-600, and R-290/R-600 are 1.66–1.96-fold, 1.28–1.38-fold and 1.57–1.88-fold greater as compared with those for R-134a under equal heat and mass fluxes. Also, the two-phase flow frictional pressure drop for R-600, R-290/R-600 and R-290 are 1.41–1.60-fold, 1.32–1.50-fold and 1.22–1.40-fold smaller as compared with that for R-134a. A new heat transfer correlation was presented by using a superposition model to predict the experimental data for both pure refrigerants and refrigerant mixtures. Experimental results were compared with several correlations which predict the evaporative heat transfer, which are in good agreement with experimental data.     

紧凑满液型叉排管束蒸发换热器内水的沸腾强化换热特性

采用紧凑满液型蒸发换热器,利用水平传热管叉排管束狭窄空间内早期沸腾强化换热机理将中小热负 荷条件下的自然对流换热转化为旺盛核沸腾换热,换热性能大大优于传统的降膜式蒸发换热器。对水平传热管 管束在受限空间内沸腾强化换热进行实验研究,确认了紧凑满液式水平管蒸发换热器具有良好的换热性能,传 热管在管束中的位置对换热特性已经没有明显影响,随着压力增加,受限空间内沸腾强化换热强化效果显著增 加。