Forced convective heat transfer to supercritical water flowing in tubes

Experimental investigations were made of heat transfer to supercritical water flowing in a horizontal tube and vertical tubes. A comprehensive set of data was obtained for pressures from 226 to 294 bar, bulk temperatures from 230 to 540°C, heat fluxes from 116 to 930 kW/m² and mass velocities from 310 to 1830 kg/m²s. Because the physical properties of supercritical fluids change rapidly with temperature in the pseudocritical region, the heat transfer coefficients show unusual behavior depending upon the heat flux. At low or modetate heat fluxes relatively to the flow rate, a satisfactory correlation was obtained, which predicts reasonably well the enhanced heat transfer coefficients near the pseudocritical point. The several characteristics of the deterioration in heat transfer which occurs at high heat fluxes were clarified, and the limit heat flux for the occurrence of the deterioration was determined in connection with the flow rate.     

Investigation on the characteristics and mechanisms of unusual heat transfer of supercritical pressure water in vertically-upward tubes

The heat transfer characteristics of supercritical pressure water in a vertically-upward optimized internally-ribbed tube was investigated experimentally to study the mechanisms of unusual heat transfer of supercritical pressure water in the so-called large specific heat region. The experimental parameters were as follows. The pressure at the inlet of the test section ranged from 22.5 to 29.0 MPa, and the mass flux of the fluid was from 650 to 1200 kg/m² s, and the heat flux on the inside wall of the tube varied from 200 to 660 kW/m². According to experimental data, the characteristics of heat transfer enhancement and also the heat transfer deterioration of supercritical pressure water in the large specific heat region was analyzed and based on the comparison and analysis of the current major theories that were used to explain the reasons for unusual heat transfer to occur, the mechanisms of heat transfer enhancement and deterioration were discussed, respectively. The enhanced heat transfer was characterized by the gently changing wall temperature, the small temperature difference between the inside-tube-wall and the bulk fluid and the high heat transfer coefficient in comparison to the normal heat transfer. The deteriorated heat transfer could be characterized by the sharply increasing wall temperature, the large temperature difference and a sudden decrease in heat transfer coefficient in comparison to the normal heat transfer. The heat transfer enhancement of the supercritical pressure water in the large specific heat region was suggested to be a result of combined effect caused by the rapid variations of thermophysical properties of the supercritical pressure water in the large specific heat region, and the same was true of the heat transfer deterioration. The drastic changes in thermophysical properties near the pseudocritical points, especially the sudden rise in the specific heat of water at supercritical pressures, might result in the occurrence of the heat transfer enhancement, while the covering of the heat transfer surface by fluids lighter and hotter than the bulk fluid made the heat transfer deteriorated eventually and explained how this lighter fluid layer formed.     

An investigation into the heat transfer characteristics of spiral wall with internal rib in a supercritical sliding-pressure operation once-through boiler

Within the pressure range of 9–28 MPa, mass velocity range of 600–1 200 kg/(m²·s), and heat flux range of 200–500 kW/m², experiments were performed to investigate the heat transfer to water in the inclned upward internally ribbed tube with an inclined angle of 19.5 degrees, a maximum outer diameter of 38.1 mm, and a thickness of 7.5 mm. Based on the experiments, it was found that heat transfer enhancement of the internally ribbed tube could postpone departure from nucleate boiling at the sub-critical pressure. However, the heat transfer enhancement decreased near the critical pressure. At supercritical pressure, the temperature difference between the wall and the fluid increased near the pseudo-critical temperature, but the increase of wall temperature was less than that of departure from nucleate boiling at sub-critical pressure. When pressure is closer to the critical pressure, the temperature difference between the wall and the fluid increased greatly near the pseudo-critical temperature. Heat transfer to supercritical water in the inclined upward internally ribbed tube was enhanced or deteriorated near the pseudo-critical temperature with the variety of ratio between the mass velocity and the heat flux. Because the rotational flow of the internal groove reduced the effect of natural convection, the internal wall temperature of internally ribbed tube uniformly distributed along the circumference. The maximum internal wall temperature difference of the tube along the circumference was only 10 degrees when the fluid enthalpy exceeded 2 000 J/g. Considering the effect of acute variety of the fluid property on heat transfer, the coreelation of heat transfer coefficient on the top of the internally ribbed tube was provided. Translated from Proceedings of CSEE, 2005, 25(16): 90–95 [译自: 中国电机工程学报]     

Experimental investigation on heat transfer characteristics of low mass flux rifled tube with upward flow

An experiment for heat transfer of water flowing in a vertical rifled tube was conducted at subcritical and supercritical pressure. The main purpose is to explore the heat transfer characteristics of the new-type rifled tube at low mass flux. Operating conditions included pressures of 12–30 MPa, mass flux of 232–1200 kg/(m² s), and wall heat fluxes of 133–719 kW/m². The heat transfer performance and wall temperature distribution at various operating conditions were captured in the experiment. In the present paper, the heat transfer mechanism of the rifled tube was analyzed, the effects of pressure, wall heat flux and mass flux on heat transfer were discussed, and corresponding empirical correlations were also presented. The experimental results exhibit that the rifled tube has an obvious enhancement in heat transfer, even at low mass flux. In comparison with a smooth tube, the rifled tube efficiently prevents Departure from Nucleate Boiling (DNB) and delays dryout at subcritical pressure, and also improves the heat transfer of supercritical water remarkably, especially near pseudo-critical point. An increase in pressure or wall heat flux impairs the heat transfer at both subcritical and supercritical pressure, whereas the increasing mass flux has a contrary effect.     

Heat transfer with water flowing upward in a tube for pressures up to supercritical region

A heat transfer experiment was conducted in a tube of 6.07 mm in diameter with water flowing upward, covering the ranges of pressure of 10–23 MPa, mass flux of 288–1298 kg/(m²·s), local water temperature of 78°C–270°C, heat flux of 0.23–1.18 MW/m² and Reynolds number of 5.5 × 10³−3.9 × 10⁴. The experimental results were compared with the predictions of the Dittus-Boelter correlation, Jackson correlation, Bishop correlation, Swenson correlation and Yamagata correlation. Significant deterioration in heat transfer was observed in both subcritical and supercritical region due to the effect of buoyancy force, but it was not predicted reasonably by the existing correlations.     

Heat transfer for flow boiling of water and critical heat flux in a half‐heated round tube under low‐pressure conditions

Heat transfer for flow boiling of water and critical heat flux (CHF) experiments in a half‐circumferentially heated round tube under low‐pressure conditions were carried out. To clarify the flow patterns in the heated section, experiments in the round tube under the same conditions were also carried out, and their results were compared. The experiments were conducted with atmospheric‐pressure water in test sections with inner diameter D = 6 mm, heated length L = 360 mm, inlet water subcooling ΔT_in = 80 K, and mass velocity G from 0 to 2000 kg/(m²·s) for the half‐circumferentially heated round tube and from 0 to 7000 kg/(m²·s) for the full‐circumferentially heated tube. The experimental data demonstrated that the wall temperature near the outlet of the half‐circumferentially heated tube remained almost the same until CHF. It was found that burnout occurred when the flow regime changed from churn flow to annular flow, and the liquid film on the heated wall dried out although liquid film on the unheated wall remained. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(3): 149–164, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10022     

An experimental investigation of convection heat transfer to supercritical carbon dioxide in miniature tubes

Experimental results of convection heat transfer to supercritical carbon dioxide in heated horizontal and vertical miniature tubes are reported in this paper. Stainless steel circular tubes having diameters of 0.70, 1.40, and 2.16 mm were investigated for pressures ranging from 74 to 120 bar, temperatures from 20 to 110 °C, and mass flow rates from 0.02 to 0.2 kg/min. The corresponding Reynolds numbers and Prandtl numbers ranged from 10⁴ to 2×10⁵ and from 0.9 to 10, respectively. It is found that the buoyancy effects were significant for all the flow orientations, although Reynolds numbers were as high as 10⁵. The experimental results reveal that in downward flow, a significant impairment of heat transfer was discerned in the pseudocritical region, although heat transfer for both horizontal and upward flow was enhanced. The experimental results further indicate that in all the flow orientations, the Nusselt numbers decreased substantially as the tube diameter shrunk to <1.0 mm. Based on the experimental data, correlations were developed for the axially-averaged Nusselt number of convection heat transfer to supercritical carbon dioxide in both horizontal and vertical miniature heated tubes.     

Numerical Investigation of Conjugate Heat Transfer to Supercritical CO2 in a Vertical Tube-in-Tube Heat Exchanger

Conjugate heat transfer to supercritical CO₂ in a vertical tube-in-tube heat exchanger was numerically investigated. The results demonstrate that most models considered are able to reproduce the heat transfer processes qualitatively, and the Abe, Kondoh, and Nagano model shows optimal agreement with the experimental data. The influences of hot fluid mass flux and temperature of the shell side, supercritical fluid mass flux of the tube side, flow direction, and pipe diameter on conjugate heat transfer were investigated based on velocity and turbulence fields. It is concluded that hot fluid mass flux and temperature of the shell side significantly affect heat transfer of the tube side. Mixed convection is the main heat transfer mechanism for the supercritical CO₂ conjugate heat transfer process when the inner diameter of the tube is greater than 1 mm. In addition, density variation is highly significant for heat transfer of supercritical CO₂ while high viscosity hinders the distortion of the flow field and reduces deterioration in heat transfer.     

New proposed two-phase multiplier and evaporation heat transfer coefficient correlations for R134a flowing at low mass flux in a multiport minichannel

New correlations of the two-phase multiplier and heat transfer coefficient of R134a during evaporation in a multiport minichannel at low mass flux are proposed. The experimental results were obtained from a test using a counter-flow tube-in-tube heat exchanger with refrigerant flowing in the inner tube and hot water in the gap between the outer and inner tubes. Test section is composed of the extruded multiport aluminium inner tube with an internal hydraulic diameter of 1.2 mm and an acrylic outer tube with an internal hydraulic diameter of 25.4 mm. The experiments were performed at heat fluxes between 10 and 35 kW/m², and a refrigerant mass flux between 45 and 155 kg/(m² s). Some physical parameters that influenced the frictional pressure drop and heat transfer coefficient are examined and discussed in detail. The pressure drop and heat transfer coefficient results are also compared with existing correlations. Finally, new correlations for predicting the frictional pressure drop and heat transfer coefficient at low mass fluxes are proposed.     

Evaporative characteristics of R-134a and R-600a in horizontal tubes with perforated strip-type inserts

Experimental heat transfer coefficients for R-134a and R-600a in horizontal tubes with vertically positioned perforated strip-type inserts are reported in this paper. Tests were conducted using a single-tube evaporator test facility. The test section used was 2000 mm long, 10.6 mm inside diameter, horizontal, smooth copper tube with perforated strip-type inserts made from the same material (copper). Test parameters were varied as follows: heat flux 9.1-31.2 kW/m²; mass velocity 82.3-603.3 kg/m² s; quality 0-0.85, and a saturation temperature of 6 °C. The flow pattern were identified for different test tubes and flow conditions. The heat transfer coefficients for R-600a were higher than those for R-134a. The heat transfer performance and pressure drop can be improved up to 2.5 and 1.5, respectively for a 96 perforated holes enhanced tube. All comparisons were based on the same nominal mass flow rate. Finally, an empirical correlation was developed.     

Numerical Investigation of Jet Impingement Cooling of a Flat Plate with Carbon Dioxide at Supercritical Pressures

Confined round jet impingement cooling of a flat plate at constant heat flux with carbon dioxide at supercritical pressures was investigated numerically. The pressure ranged from 7.8 to 10.0 MPa, which is greater than the critical pressure of carbon dioxide, 7.38 MPa. The inlet temperature varied from 270 to 320 K and the heat flux ranged from 0.6 to 1.6 MW/m². The shear-stress transport turbulence model was used and the numerical model was validated by comparison with experimental results for jet impingement heating with hot water at supercritical pressures. Radial conduction in the jet impingement plate was also considered. The sharp variations of the thermal-physical properties of the fluid near the pseudocritical point significantly influence heat transfer on the target wall. For a given heat flux, the high specific heat near the wall for the proper inlet temperature and pressure maximizes the average heat transfer coefficient. For a given inlet temperature, the heat transfer coefficient remains almost unchanged with increasing surface heat flux at first and then decreases rapidly as the heat flux becomes higher due to the combined effects of the thinner high specific heat layer and the smaller thermal conductivity at higher temperature.     

Characteristics of heat transfer inside a tube during sodium‐water reaction in a FBR steam generator

Sodium reacts chemically with water in the case of an unexpected tube failure of a steam generator (SG) in a fast breeder reactor (FBR). In order to predict the event with high accuracy, it is very important to understand the characteristics of heat transfer inside the tube in detail during the tube failure due to the sodium–water reaction. Experiments were performed by using purified water under the following conditions: initial pressure of 11.2–13.4 MPa, initial water temperature of 200 °C, and water mass flux of 45.7 to 3630 kg/(m²s). The test tube was heated rapidly by high‐frequency induction current. The time averaged heat flux was estimated by using an inverse solution from the measured temperatures at two points on three different locations along the tube. It was confirmed that the derived values agreed with the measured heat fluxes on the outer surface within 20% accuracy. It was found that the characteristics of the heat transfer strongly depend on the flow rate. The heat transfer on the wall changed from nucleate boiling to transient‐film boiling during increasing the heat flux and returned to the nucleate boiling during decreasing the heat flux. A counterclockwise cycle always appeared in the transition boiling region, where the nucleate and film boiling coexisted and the area ratio of these varied with time. The adequacy of heat transfer correlations to evaluate tube overheating was confirmed. © 2010 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com ). DOI 10.1002/htj.20320     

Flow boiling in a 1.1 mm tube with R134a: Experimental results and comparison with model

A detailed comparison of flow boiling heat transfer results in a stainless steel tube of 1.1 mm internal diameter with results of a three-zone flow model are presented in this paper. The working fluid is R134a. Other parameters were varied in the range: mass flux 100–600 kg/m² s; heat flux 16–150 kW/m² and pressure 6–12 bar.The experimental results demonstrate that the heat transfer coefficient increases with heat flux and system pressure, but does not change with vapour quality when the quality is less than about 50% for low heat and mass flux values. The effect of mass flux is observed to be insignificant. For vapour quality values greater than 50% and at high heat flux values, the heat transfer coefficient does not depend on heat flux and decreases with vapour quality. This could be caused by dryout. The three-zone evaporation model predicts the experimental results fairly well, especially at relatively low pressure. However, the dryout region observed at high quality is highly over-predicted by the model. The sensitivity of the performance of the model to the three optimised parameters (confined bubble frequency, initial film thickness and end film thickness) and some preliminary investigation relating the critical film thickness for dryout to measured tube roughness are also discussed.     

Forced convective boiling of subcooled water at high velocity

This paper deals with heat transfer and critical heat flux (CHF) in subcooled flow boiling offering a fundamental study aimed at high heat flux cooling. Experiments with water at 0.12 MPa were conducted in a mass velocity range from 500 kg/m²s to 15,000 kg/m²s (velocity from 0.5 m/s to 15 m/s) and subcooling from 20 K to 60 K. A sheet of stainless steel (80 mm in heated length, 10 mm wide, and 0.2 mm thick) was mounted flush with a sidewall of a vertical rectangular channel (cross-section 20 mm by 30 mm) and heated directly using direct current. It was found that mass velocity and subcooling strongly affect CHF and heat transfer in non-boiling convection and partial nucleate boiling regimes. These two parameters have no appreciable influence in the fully developed nucleate boiling regime. In the parameter range used, CHF reached 15 MW/m². Boiling bubble behavior just prior to reaching CHF was found to vary depending on mass velocity and subcooling. 1998 Scripta Technica, Heat Trans Jpn Res, 27(5): 376–389, 1998     

Experimental and numerical study on heat transfer and flow characteristics in an alternating cross‐section flattened tube

An experimental and numerical study on convection heat transfer of water flowing through an alternating cross‐section flattened (ACF) tube are investigated in this paper. The thermal‐fluid characteristics were evaluated by numerical simulation. The test run conditions covered a mass flux of 200 to 800 kg m^?2 s^?1, a heat flux of 10 kW/m², and an inlet temperature of 40°C. The results showed that the Nusselt number increased with the increase in mass flux. Moreover, the heat transfer was also affected by the flow characteristics. Vortices were formed at the curved wall, and their intensities were increased along the flow direction. It was also found that the heat transfer and pressure drop were larger than that of the circular tube. However, the thermal performance was greater than the pressure loss penalty. The comparison results showed that the ACF tube had better performance than the circular tube. Further, the details of heat transfer, flow resistance, and fluid behavior were investigated and discussed in this study.     

Boiling Heat Transfer and Pressure Drop of a Refrigerant R32 Flowing in a Small Horizontal Tube

In this study, experiments were performed to examine characteristics of flow boiling heat transfer and pressure drop of a low global warming potential refrigerant R32 flowing in a horizontal copper circular tube with 1.0 mm inside diameter for the development of a high-performance heat exchanger using small-diameter tubes or minichannels for air conditioning systems. Axially local heat transfer coefficients were measured in the range of mass fluxes from 30 to 400 kg/(m²·s), qualities from 0.05 to 1.0, and heat fluxes from 2 to 24 kW/m² at the saturation temperature of 10°C. Pressure drops were also measured in the rage of mass fluxes from 30 to 400 kg/(m²·s) and qualities from 0.05 to 0.9 at the saturation temperature of 10°C under adiabatic condition. In addition, two-phase flow patterns were observed through a sight glass fixed at the tube exit with a digital camera. The characteristics of boiling heat transfer and pressure drop were clarified based on the measurements and the comparison with data of R410A obtained previously. Also, measured heat transfer coefficients were compared with two existing correlations.     

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.     

Numerical study on in-tube laminar heat transfer of supercritical fluids

Numerical calculations were carried out for supercritical carbon dioxide flowing in miniature tubes with Re less than 1000. The heat transfer coefficient α and friction factor f were numerically studied for different values of the tube diameter, pressure, mass flux, and heat flux. When compared with the constant property flow, where Nu = 4.364 and f = 64/Re for a circular tube under a constant heat flux condition, a large divergence from the constant value was obtained for both Nu and f·Re in the vicinity of the pseudocritical temperature T_m. When cooled under a constant heat flux, Nu attained its peak value when T_b > T_m and its minimum value when T_b < T_m, while f·Re attained its peak value at T_b = T_m. With regard to the heating process, the reverse tendencies were confirmed. The variations of the specific heat with temperature were found to be the dominant factor for Nu. In addition, empirical correlations that considered the cross-sectional distribution of thermophysical properties were proposed to predict the values of Nu and f both in the near-pseudocritical temperature region and in the thermal entrance region of the tube. The proposed correlations were also verified by comparing the predicted results with numerical results obtained by using supercritical water.     

Correlation of the Flow Pattern and Flow Boiling Heat Transfer in Microchannels

Flow boiling in microchannels is characterized by the considerable influence of capillary forces and constraint effects on the flow pattern and heat transfer. In this article we utilize the features of gas–liquid flow patterns in rectangular microchannels under adiabatic conditions to explain the regularities of refrigerants flow boiling heat transfer. The flow-pattern maps for the upward and horizontal nitrogen–water flow in a microchannel with the size of 1500 × 720 μm were determined via dual-laser flow scanning and compared with corrected Mishima and Ishii prediction. Flow boiling heat transfer was studied for vertical and horizontal microchannel heat sink with similar channels using refrigerants R-21 and R-134a. The data on local heat transfer coefficients were obtained in the range of mass flux from 33 to 190 kg/m²-s, pressure from 1.5 to 11 bar, and heat flux from 10 to 160 kW/m². The nucleate and convective flow boiling modes were observed for both refrigerants. It was found that heat transfer deterioration occurred for annular flow when the film thickness became small to suppress nucleate boiling. The mechanism of heat transfer deterioration was discussed and a model of heat transfer deterioration was applied to predict the experimental data.     

Two-Phase Frictional Pressure Drop and Flow Boiling Heat Transfer for R245fa in a 2.32-mm Tube

Experimental two-phase frictional pressure drop and flow boiling heat transfer results are presented for a horizontal 2.32-mm ID stainless-steel tube using R245fa as working fluid. The frictional pressure drop data was obtained under adiabatic and diabatic conditions. Experiments were performed for mass velocities ranging from 100 to 700 kg m^?2 s^?1, heat flux from 0 to 55 kW m^?2, exit saturation temperatures of 31 and 41°C, and vapor qualities from 0.10 to 0.99. Pressures drop gradients and heat transfer coefficients ranging from 1 to 70 kPa m^?1 and from 1 to 7 kW m^?2 K^?1 were measured. It was found that the heat transfer coefficient is a strong function of the heat flux, mass velocity, and vapor quality. Five frictional pressure drop predictive methods were compared against the experimental database. The Cioncolini et al. (2009) method was found to work the best. Six flow boiling heat transfer predictive methods were also compared against the present database. Liu and Winterton (1991), Zhang et al. (2004), and Saitoh et al. (2007) were ranked as the best methods. They predicted the experimental flow boiling heat transfer data with an average error around 19%.