Convective Boiling Heat Transfer Research Articles

Experimental study on convective boiling heat transfer for pure refrigerants and refrigerant mixtures in a horizontal tube

Boiling heat transfer coefficients of pure refrigerants (R22, R32, R134a, R125, R290, and R600a) and refrigerant mixtures (R32/Rl34a, R290/ R600a, and R32/R125) are measured experimentally and compared with Chen's correlation. The test section is a seamless stainless steel tube with inner diameter of 7.7mm and uniformly heated by applying electric current directly to the tube. Heat fluxes range from 10 to 30kW. Mass fluxes are set to 424 ~ 742kg/ms for R22, R32, R134a, R32/R134a, and R32/Rl25 ; 265 ~ 583kg/ms for R290, R600a, and R290/R600a. Heat transfer coefficients depend strongly on heat flux at a low quality region and become independent as quality increases. Convective boiling term in the Chen's correlation predicts experimental data of the pure refrigerants fairly well (relative error of 12.1% for the data of quality over 0.2). The correlation for pure substances overpredicts the heat transfer coefficients for nonazeotropic refrigerant mixtures.

A Correlation for Forced Convective Boiling Heat Transfer of Binary Refrigerant Mixtures. 2nd Report. A Spirally Grooved Tube.

A correlation is developed for evaporation heat transfer inside a spirally grooved tube, and compared with the experimental data. First, heat transfer coefficients for pure refrigerants were correlated in a form such that two-phase flow pattern was taken into account. Next, a correlation for binary mixtures was derived from the correlation for pure refrigerants and the thermodynamic relations for mixtures. This is based on the model described in the previous report. The predicted heat transfer coefficients agreed with the experimental data with a mean deviation of 7.4% and 8.8% for pure refrigerants (R11, R114, R123) and binary mixtures (R11/R114, R123/R134a), respectively.

Boiling heat transfer in small horizontal tube bundles at low cross-flow velocities

An experimental investigation has been carried out to determine the local forced convective boiling heat transfer coefficient in small tube bundles consisting of horizontal tubes in a vertical column arranged in a large channel under low cross-flow velocities in saturated distilled water at atmospheric pressure. The effects of the impressed heat flux, cross-flow velocity and tube geometry on the heat transfer characteristics have been investigated. While no significant effect of the bundle geometry was observed on the heat transfer coefficient of the bottom tube compared to that of the single tube, vapour bubbles rising from the lower tubes and agitating the liquid around the upper tubes were found to yield higher values for heat transfer coefficients on the upper tubes. A Chen-type relation has been used to correlate the data on local forced convective heat transfer coefficients of upper tubes with reasonably accuracy.

A Correlation for Forced Convective Boiling Heat Transfer of Binary Refrigerant Mixtures.

A correlation, which does not involve any empirical constants dependent on refrigerants, is developed for forced convective boiling heat transfer inside a smooth tube. The correlation is based on the model where the heat transfer coefficient of a mixture is lower than that of the equivalent pure fluid due to the mixture effects on nucleate boiling and to the heat transfer resistance in the vapor phase. The predicted heat transfer coefficients agreed with the experimental data with mean deviation of 5.9 and 6.5% for pure refrigerants (R11, R114, R123, R12) and binary mixtures (R11/R114, R123/R134a, R11/R12), respectively.

A Correlation for Forced Convective Boiling Heat Transfer of Refrigerants in a Microfin Tube

A Correlation for Forced Convective Boiling Heat Transfer of Refrigerants in a Microfin Tube

A theoretical and experimental study on subcooled flow boiling at high heat flux

In a subcooled flow boiling system at high heat flux, the major heat transfer mechanism places emphasis on a very thin liquid layer, known as the ‘sublayer’ which is trapped between the heated surface and the vapor blankets. Base on the convective boiling heat transfer dominated by the heat conduction through the liquid sublayer, a theoretical model for subcooled flow boiling heat transfer has been developed. To provide useful data in the simulation of Light Water Reactors (LWRs) conditions, heat transfer experiments for up-flow boiling water through a vertical tube at the pressure ranging from 6.9 to 15.5 MPa have been conducted. The experimental results are compared with the predictions of the present model and other five famous correlations. For the LWRs subcooled flow boiling, the comparison reveals that the present model show the best agreement with the measured data.

Forced convective boiling in binary mixtures

ENEA started in 1990 a new research programme on ‘Thermal Hydraulics Of Mixtures’ (THOM) focused on the heat transfer in forced convective boiling of refrigerant mixtures. The main aim of the research is to accomplish new experimental data on binary mixtures in forced convective boiling and particularly on three main topics: convective boiling heat transfer, hysteresis in nucleate boiling incipience and critical heat flux. A first preliminary step of the research, started in the middle of 1990, consisted in the thermal hydraulic characterization of the pure components of the binary mixture chosen, namely R 12 (CCl 2F 2) and R 114 (C 2Cl 2F 4). At the end of 1991 (and still in progress) the main investigation on the heat transfer performance of different compositions of an R 12/R 114 mixture started. In the present paper, a review of previous works, together with a description of the first experimental results on heat transfer coefficients is given. Finally a comparison of experimental data with the available correlations, is accomplished.

A correlation for forced convective boiling heat transfer of nonazeotropic refrigerant mixture of HCFC22/CFC114 in a horizontal smooth tube

Experiments are carried out on the boiling heat transfer of nonazeotropic refrigerant mixtures of HCFC22/CFC114 flowing inside a 7.9 mm ID horizontal smooth tube, Using a water-heated, double-tube type evaporator, the local heat transfer coefficients are measured for both counter and parallel flows. An empirical correlation equation is proposed for the data in the annular-flow regime, taking into account the mixture effect in the authors' previous correlation equation for pure refrigerants. The mean deviation between the calculated and measured heat transfer coefficients is 8.9% for the present experimental data and 7.1% for the data obtained by Jung. The equation reveals that the convection heat transfer is dominant due to the suppression of nucleate boiling for the mixtures.

A correlation for forced convective boiling heat transfer of pure refrigerants in a horizontal smooth tube

An experimental study is reported on the boiling heat transfer of HFC134a, HCFC22, CFC114 and CFC12 flowing inside a 7.9 mm ID horizontal smooth tube. Using a water-heated, double-tube type evaporator, the local heat transfer coefficients are measured for both counter and parallel flows. Based on the supposition of Chen that the total heat flux is represented as the sum of forced convective contribution and nucleate boiling contribution, a correlation equation is proposed for the data in the annular-flow regime. The mean deviation between the calculated and measured heat transfer coefficients is 12.2% for the present experimental data and 9.5% for the data available from literature. The proposed correlation shows that the nucleate boiling is not fully suppressed even in the high-quality region in the case of counter flow, while convective evaporation is dominant in the high-quality region with uniform heat flux condition.

Heat Transfer Characteristics of an Earth Probe Model for a Ground-Source Heat Pump. Experiments on Forced Convection Boiling Heat Transfer in a Vertical Double Tube:the Case of Small Flow Rate.

An experimental investigation was conducted to determine the heat transfer characteristics of HCFC-22 for a two-phase double tube connected to a heat pump. This study focussed on the forced convection boiling heat transfer of small flow rate in a double tube. It was clarified that the quality at the dry-out point increases as mass velocity increases, and that the heat transfer coefficient of two-phase flow is about 30 times as large as that of the liquid single-phase laminar flow. Furthermore, it was found that the heat transfer performance of the double tube is superior to that of a single tube.

AN EXPERIMENTAL STUDY OF POST-CHF HEAT TRANSFER IN A 3x3 ROD BUNDLE

This chapter describes post-critical heat flux (CHF) heat transfer experiments in a 3×3 rod bundle. The objective of these experiments was to obtain measurements of thermodynamic nonequilibrium in the post-CHF regime and to characterize its effects on two-phase heat transfer. In boiling heat transfer systems a maximum value of heat flux called critical heat flux occurs when the heated surface can no longer support continuous liquid contact. A two-phase heat transfer loop was designed and built specifically for convective boiling heat transfer studies using steam-water mixtures. Each fuel rod simulator was equipped with 12 thermocouples such that two thermocouples were in the lower extension, one thermocouple in the upper extension and the remaining nine were in the test length. The data acquisition system consisted of a HP series 200, 9816A computer, HP 3497A Data Acquisition/Control unit and an HP34S6A Digital Voltmeter with all the devices on a single HP-IB interface bus, and a Bascom-Turner fast multichannel data recorder.

A Correlation for Forced Convective Boiling Heat Transfer of Nonazeotropic Refrigerant Mixtures of HCFC22/CFC114 in a Horizontal Smooth Tube.

An experimental study is reported on convective boiling heat transfer for nonazeotropic refrigerant mixtures of HCFC22/CFC114 inside a horizontal smooth tube. The test evaporator is of the double-tube type in which the heating water flows in the outer annulus, and the local heat transfer coefficients are measured for both counter and parallel flow. Based on the equation, which correlates the heat transfer coefficient for pure refrigerants as the sum of forced convective contribution acv and nucleate boiling contribution anb, an empirical correlation equation for the mixture is developed with the data corresponding to the annular flow pattern. The mean deviation between the calculated and measured heat transfer coefficients is 8.9% for the present experimental data and 7.1% for data obtained by Jung. The equation reveals that the contribution of the nucleate boiling to the total heat flux is much smaller than that in pure refrigerants, while the convective heat transfer coefficient is almost the same.

A Correlation for Forced Convective Boiling Heat Transfer of Single-Component Refrigerants in a Horizontal Smooth Tube.

An experimental study is reported on convective boiling heat transfer for HFC134a, HCFC22, CFC114 and CFC12 inside a horizontal smooth tube. The test evaporator is of the double-tube type in which the heating water flows in the outer annulus, and the local heat transfer coefficients are measured for both counter and parallel flow. Based on the supposition of Chen that the total heat flux q is represented as the sum of forced convective contribution qcv and nucleate boiling contribution qnb, a general correlation is developed for the data in the annular flow regime. The mean deviation between the calculated and measured heat transfer coefficients is 12.2% for the present experimental data and 9.5% for available data collected from previous research. According to the proposed correlation, nucleate boiling is not suppressed even in the high-quality region in the case of counter flow, while two-phase forced convection has been recognized as predominant in the high-quality region with uniform heat flux conditions.

Experimental results of maximum transition boiling temperature during upflow in a circular tube at medium pressure

Temperature-controlled measurements of forced convective boiling heat transfer during upflow of water in a vertically mounted short circular tube were performed between saturation and 400°C wall temperature. From these experimental results, 343 data points of the maximum transition boiling temperature, which corresponds to the minimum heat flux condition, were deduced. The parametric effects of pressure (range 0.11–1.2 MPa), local quality (−0.04–0.8), mass flux [10−1000 kg/(m 2·s)], and inlet subcolling (5–57 K) as comprised in these data are discussed. The present data are compared with experimental data and correlations of the true quench temperature available from the literature. Extrapolating the data to zero flow and comparing the resultant pseudodata with pertinent theories and experimental data for the minimum film boiling temperature reveals fair agreement over the entire pressure range with recent data by Sakurai et al, only.

Heat Transfer Enhancement by Bubble Formation near the Heating Surface.

The convective boiling heat transfer of an immiscible binary liquid mixture has been investigated by using water as a main fluid. R113 and R11 were atomized into the water as volatile material. When the boiling of the volatile material occurs, the heat transfer is cohanced by the bubble formation near the heating surface. The heat transfer enhancement occurs by the addition of a small amount of volatile material, and is insensitive to further increases in its concentration. The enhancement is remarkable in the case of low Reynolds number. The heat transfer enhancement was also observed under the condition that gas bubbles were generated on the heating surface by electrolysis, and it was clarified that the enhancement is caused by the agitation effect of bubbles near the wall.

A comparison of the flow boiling performance characteristics of partially-heated cross-ribbed channels with different rib geometries

Measured local heat transfer data and the results of flow visualization experiments are reported for convective boiling of R-113 and methanol in vertical cross-ribbed channels similar to geometries used in formed-plate compact heat exchangers. Experiments are conducted using a test section in which one wall of the channel is heated while the opposite wall is adiabatic and transparent to allow visual observation of the flow. Heat transfer data are obtained for three different rib crossing patterns at coolant mass flux values between 40 and 230 kg m −2 s −1 and mass qualities between 0.10 and 0.80. Comparison of these data, together with those for a fourth geometry previously reported, indicate that variations of the rib spacing and angle have a significant effect on boiling heat transfer performance. Variation of the rib crossing pattern in these channels is found to have only a small effect on heat transfer. The swirl and agitation induced by the ribs are found to increase the convective boiling heat transfer coefficient by as much as a factor of 4 above that for a round tube under comparable flow conditions. Methods of correlating the heat transfer data for annular film-flow boiling in these geometries are also discussed.

Full-Core Test Method for Experimental Determination of Convective Boiling Heat Transfer Coefficients in Tubes of Crossflow Compact Evaporators

An experimental methodology is proposed in which localized convective boiling heat transfer coefficients inside the tubes of compact evaporators are determined by testing of full evaporator cores. The proposed technique makes use of a special test system having two main flow circuits. One of these flow loops is a conventional vapor compression system, which provides a steady, low-quality, two-phase flow of refrigerant to the tube side of the evaporator. The second primary flow loop provides a steady flow of the vapor of a second working fluid, which condenses on the finned side of the evaporator. Measured data from this system are analyzed using an iterative scheme. Trends in the variation of the refrigerant-side heat transfer coefficient determined by this method throughout a typical evaporator core are described, and the differences and similarities relative to previously published results for single round tubes are discussed.

An Experimental Study of Convective Boiling in a Partially Heated Horizontal Channel With Offset Strip Fins

Measured local heat transfer data and visually observed two-phase flow behavior are reported for convective boiling of methanol and water in a partially heated horizontal channel with offset strip fins. In this study, a special test section was used that permitted direct visual observation of the two-phase flow while simultaneously measuring the convective boiling heat transfer coefficient at several locations along the channel. One wall of the channel was heated while the opposite and lateral walls were adiabatic. Measured local heat transfer coefficients on the heated portion of the channel wall were obtained for top heating and bottom heating of the horizontal channel. For the conditions examined here, the two-phase flow was in the stratified, wavy, or annular flow regime over almost the entire length of the channel. It was visually observed that virtually no nucleate boiling is present when the flow is in these regimes. A flow regime map is presented, which summarizes the observed two-phase flow characteristics. At the same quality and mass flux levels, the measured heat transfer coefficients for top heating and bottom heating were found to be virtually the same for the conditions examined here. In addition for the same quality and mass flux, the convective boiling heat transfer coefficients for top heating and bottom heating agree well with the results for comparable experiments with the channel oriented vertically. Hence, prior to dryout of the liquid film, the boiling heat transfer performance appears to be insensitive to channel orientation. Observed differences in the dryout conditions for top heating and bottom heating are also discussed.

Heat transfer and two-phase flow during convective boiling in a partially-heated cross-ribbed channel

Measured local heat transfer data and visual observations of the two-phase flow behavior are reported for convective boiling of saturated liquids in a cross-ribbed channel similar to geometries used in formed-plate compact heat exchangers. Experiments in this study were conducted using a special test section which permitted direct visual observation of the boiling process while simultaneously measuring the local heat transfer coefficient at several locations along the channel. One wall of the channel was heated while the opposite and lateral walls were adiabatic. Measured local heat transfer coefficients on the heated portion of the channel wall were obtained for convective boiling of methanol and n-butanol at atmospheric pressure with the channel oriented vertically and in horizontal positions with top heating, side heating and bottom heating of the channel. Vertical flows were observed to be in the churn or annular flow regimes over most of the channel length whereas the horizontal flows were either in the wavy or annular flow regime over most of the channel. Visual observations also indicated that virtually no nucleate boiling was present when the flow was in one of these three regimes. For the same coolant and flow conditions, at moderate to high qualities, the measured convective boiling heat transfer coefficients for the vertical and horizontal orientations were usually found to differ by only a small amount. However, for some orientations, partial dryout of the heated wall of the channel was sometimes observed to reduce the heat transfer coefficient. A method of correlating the heat transfer data for annular film-flow boiling in cross-ribbed channel geometries is also described.

Boiling heat transfer to an impinging jet spurted into a narrow space. (1st report Space with an open end)

An experimental study has been made on the forced convection boiling heat transfer in a narrow space The heated surface diameter was 20.0mm. The distance between the nozzle plate and the heated surface was changed within a range from 0.3 mm to 0.6 mm and the nozzle diameter was 2.2 mm. Saturated water was impinged on the center of the heated surface and flowed radially on it. In this report the nozzle plate had a flat bottom which was parallel to the heated surface. It is clarified in the present study that heat transfer characteristics varied with the change of space and mass flow-rate of the impinging jet, especially in the region of transition boiling. The quantity of maximum heat flux increased as the space decreased.