Experimental study of convective condensation in an inclined smooth tube. Part I: Inclination effect on flow pattern and heat transfer coefficient

An experimental study of convective condensation of R134a in an 8.38 mm inner diameter smooth tube in inclined orientations is presented. This article, being the first of a two-part paper (the second part concentrates on the pressure drops and void fractions), presents flow patterns and heat transfer coefficients during condensation for different mass fluxes and vapour qualities for the whole range of inclination angles (from vertical downwards to vertical upwards). The results were compared with three flow pattern maps available in literature. It was found that for low mass fluxes and/or low vapour qualities, the flow pattern is strongly dependent on the inclination angle whereas it remains annular for high mass fluxes and high vapour qualities, whatever the tube orientation. The models of flow pattern maps available in the literature did not predict the experimental data well. In the inclination-dependent zone, experiments showed that there is an optimum inclination angle that leads to the highest heat transfer coefficient for downward flow. The heat transfer coefficient is strongly affected by the liquid and vapour distributions and especially by the liquid thickness at the bottom of the tube for stratified flows. Thus developing a mechanistic model of flow pattern maps is the first step in achieving a predictive tool for the heat transfer coefficient in convective condensation in inclined tubes.     

An Experimental Study of Carbon Dioxide Condensation in Mini Channels

In this study, the local characteristics of pressure drop and heat transfer were investigated experimentally for carbon dioxide condensation in a multi-port extruded aluminum test section, which had 10 circular channels each with 1.31 mm inner diameter. The CO2 was cooled with cooling water flow inside the copper blocks that were attached at both sides of the test section. The temperatures at the outer surface of the test section were measured with 24 K-type thermocouples embedded in the upper and lower surfaces along the length. Local heat fluxes were measured with 12 heat flux sensors to estimate the local enthalpies, temperatures and heat transfer coefficients. Bulk mean temperatures of CO2 at the inlet and outlet of the test section were measured with 2 K-type thermocouples. The measurements were performed for the pressure ranged from 6.48 to 7.3 MPa, inlet temperature of CO2 from 21.63 to 31.33℃, heat flux from 1.10 to 8.12 kW/m2, mass velocity from 123.2 to 315.2 kg/m2s, and vapor quality from 0 to 1. The results indicate that pressure drop is very small along the test section, heat transfer coefficient in the two-phase region is higher than that in the single-phase, and mass velocity has important effect on condensation heat transfer characteristics. In addition, experimental data were compared with previous correlations and large discrepancies were observed.     

Visualization investigation on flowing condensation in horizontal small channels with liquid separator

A simultaneous visualization and measurement experiment was carried out to investigate condensation flow patterns and condensing heat transfer characteristics of refrigerant R141b in parallel horizontal multi-channels with liquid-vapor separator. The hydraulic diameter of each channel was 1.5 mm and the channel length was 100 mm. The refrigerant vapor flowing in the small channels was cooled by cooling water. The parallel horizontal multi-channels were covered with a transparent silica glass for visualization of flow patterns. Experiments were performed at different inlet superheat temperatures(ranging from 3℃ to 7℃). Mass velocity was in the range of 82.37 kg m~(-2)s~(-1) to 35.56 kg m~(-2)s~(-1). It was found that there were three different flow patterns through the multi-channels with the increase of mass velocity. The flow patterns in each channel pass almost tended to be same and all of them were annular flows. The efficiency of the liquid-vapor separator with U-type was related to vapor mass velocity and the pressure in the small channels. It was also found that the heat transfer coefficient increased with the increase of the mass velocity while the cooling water mass flow rate increased. It increased to a top point and then decreased. It increased with the increase of superheat in the low superheat temperature region.     

Experimental analysis for the determination of the convective heat transfer coefficient by measuring pressure drop directly during annular condensation flow of R134a in a vertical smooth tube

This study investigated the direct relationship between the measured condensation pressure drop and convective heat transfer coefficient of R134a flowing downward inside a vertical smooth copper tube having an inner diameter of 8.1 mm and a length of 500 mm during annular flow. R134a and water were used as working fluids on the tube side and annular side of a double tube heat exchanger, respectively. Condensation experiments were performed at mass fluxes of 260, 300, 340, 400, 456 and 515 kg m⁻² s⁻¹ in the high mass flux region of R134a. The condensing temperatures were around 40 and 50 °C; the heat fluxes were between 10.16 and 66.61 kW m⁻². Paliwoda’s analysis, which focused mainly on the determination of the two-phase flow factor and two-phase length of evaporators and condensers, was adapted to the in-tube condensation phenomena in the test section to determine the condensation heat transfer coefficient, heat flux, two-phase length and pressure drop experimentally by means of a large number of data points obtained under various experimental conditions.     

Pressure Drop,Boiling Heat Transfer and Flow Patterns during Flow Boiling in a Single Microchannel

The boiling heat transfer and two-phase pressure drop of water in a microscale channel were experimentally investigated. The tested horizontal rectangular microchannel had a hydraulic diameter of 100 μ m and length of 40 mm. A series of microheaters provided heat energy to the working fluid, which made it possible to control and measure the local thermal conditions in the direction of the flow. Both the microchannel and microheaters were fabricated using a micro-electro-mechanical systems (MEMS) technique. Flow patterns were obtained from real-time flow visualizations made during the flow boiling experiments. Tests were performed for mass fluxes of 90, 169, and 267 kg/m²s and heat fluxes from 200 to 500 kW/m². The effects of the mass flux and vapor quality on the local flow boiling heat transfer coefficient and two-phase frictional pressure gradient were studied. The evaluated experimental data were compared with existing correlations. The experimental heat transfer coefficients were nearly independent of the mass flux and vapor quality. Most of the existing correlations did not provide reliable heat transfer coefficient predictions for different vapor quality values, nor could they predict the two-phase frictional pressure gradient except under some limited conditions.     

Flow Patterns and Heat Transfer for Flow Boiling in Small to Micro Diameter Tubes

An overview of the recent developments in the study of flow patterns and boiling heat transfer in small to micro diameter tubes is presented. The latest results of a long-term study of flow boiling of R134a in five vertical stainless-steel tubes of internal diameter 4.26, 2.88, 2.01, 1.1, and 0.52 mm are then discussed. During these experiments, the mass flux was varied from 100 to 700 kg/m²s and the heat flux from as low as 1.6 to 135 kW/m². Five different pressures were studied, namely, 6, 8, 10, 12, and 14 bar. The flow regimes were observed at a glass section located directly at the exit of the heated test section. The range of diameters was chosen to investigate thresholds for macro, small, or micro tube characteristics. The heat transfer coefficients in tubes ranging from 4.26 mm down to 1.1 mm increased with heat flux and system pressure, but did not change with vapor quality for low quality values. At higher quality, the heat transfer coefficients decreased with increasing quality, indicating local transient dry-out, instead of increasing as expected in macro tubes. There was no significant difference between the characteristics and magnitude of the heat transfer coefficients in the 4.26 mm and 2.88 mm tubes but the coefficients in the 2.01 and 1.1 mm tubes were higher. Confined bubble flow was first observed in the 2.01 mm tube, which suggests that this size might be considered as a critical diameter to distinguish small from macro tubes. Further differences have now been observed in the 0.52 mm tube: A transitional wavy flow appeared over a significant range of quality/heat flux and dispersed flow was not observed. The heat transfer characteristics were also different from those in the larger tubes. The data fell into two groups that exhibited different influences of heat flux below and above a heat flux threshold. These differences, in both flow patterns and heat transfer, indicate a possible second change from small to micro behavior at diameters less than 1 mm for R134a.     

Subcooled flow boiling heat transfer of R-134a and the associated bubble characteristics in a vertical plate heat exchanger

Subcooled flow boiling heat transfer characteristics of refrigerant R-134a in a vertical plate heat exchanger (PHE) are investigated experimentally in this study. Besides, the associated bubble characteristics are also inspected by visualizing the boiling flow in the vertical PHE. In the experiment two vertical counterflow 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 subcooled refrigerant R-134a in one channel receives heat from the downflow of hot water in the other channel. The effects of the boiling heat flux, refrigerant mass flux, system pressure and inlet subcooling of R-134a on the subcooled boiling heat transfer are explored in detail. The results are presented in terms of the boiling curves and heat transfer coefficients. The measured data showed that the slopes of the boiling curves change significantly during the onset of nucleate boiling (ONB) especially at low mass flux and high saturation temperature. Besides, the boiling hysteresis is significant at a low refrigerant mass flux. The subcooled boiling heat transfer coefficient is affected noticeably by the mass flux of the refrigerant. However, increases in the inlet subcooling and saturation temperature only show slight improvement on the boiling heat transfer coefficient.The photos from the flow visualization reveal that at higher imposed heat flux the plate surface is covered with more bubbles and the bubble generation frequency is substantially higher, and the bubbles tend to coalesce to form big bubbles. But these big bubbles are prone to breaking up into small bubbles as they move over the corrugated plate, producing strong agitating flow motion and hence enhancing the boiling heat transfer. We also note that the bubbles nucleated from the plate are suppressed to a larger degree for higher inlet subcooling and mass flux. Finally, empirical correlations are proposed to correlate the present data for the heat transfer coefficient and the bubble departure diameter in terms of boiling, Froude, Reynolds and Jakob numbers.     

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.     

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.     

Boiling Heat Transfer for Freon R21 in Rectangular Minichannel

In this paper, we study the boiling heat transfer of upward flow of R21 in a vertical mini-channel with a size of 1.6 × 6.3 mm. The heat transfer coefficient was measured as a function of heat flux for a wide range of vapor quality and for two levels of mass flow rate, G = 215 kg/m²s and G = 50 kg/m²s. The standard deviation of wall superheat over channel perimeter and in time was determined from the measurement of the wall temperature along the channel perimeter. Different heat transfer mechanisms were revealed depending on flow patterns. The main heat transfer mode for large mass flux is convective boiling. We also figure out the mode when the evaporation of thin liquid films makes the essential contribution to heat transfer. The modified models of Liu & Winterton and Balasubramanian & Kandlikar describe the experimental data well for regime when the convective boiling makes the main contribution to the heat transfer.     

Experimental Study on Flow Boiling of Deionized Water in a Horizontal Long Small Channel

In this paper, an experimental investigation on the flow boiling heat transfer in a horizontal long mini-channel was carried out. The mini-channel was with 2 mm wide and 1 mm deep and 900 mm long. The material of the mini-channel was stainless. The working fluid was deionized water. The experiments were conducted with the conditions of inlet pressure in the range of 0.2~0.5 MPa, mass flux in the range of 196.57-548.96 kg/m²s, and the outlet vapor quality in the range of 0.2 to 1. The heat flux was in the range of 292.86 kW/m² to 788.48 kW/m², respectively. The influences of mass flux and heat flux were studied. At a certain mass flow rate, the local heat transfer coefficient increased with the increase of the heat flux. If dry-out occurred in the mini-channel, the heat transfer coefficient decreased. At the same heat flux, the local heat transfer coefficient would depend on the mass flux. It would increase with the mass flux in a certain range, and then decrease if the mass flux was beyond this range. Experimental data were compared with the results of previous studies. Flow visualization and measurements were conducted to identify flow regime transitions. Results showed that there were eight different kinds of flow patterns occurring during the flow boiling. It was found that flow pattern had a significant effect on heat transfer.     

Study of flow boiling characteristics of R134a in annulus of enhanced surface tubing

The paper presents an experimental study of the flow-boiling heat-transfer characteristics of R12 and R134a in the annulus of a horizontal enhanced-surface-tubing evaporator. The test section has an inner-tube bore diameter of 17.5 mm, an envelope diameter of 28.6 mm and an outer smooth tube of 32.3 mm inside diameter. The ranges of heat flux and mass velocity covered in the tests were 5–25 kW/m² and 180–290 kg/m²/s, respectively, at a pressure of 365 kPa. In order to establish the flow regime conditions at the inlet to the test section, the test rig allows for the visualization of refrigerant flow through the preheater. The experiments show two regions of heat transfer: a nucleate boiling region where the heat transfer depends mainly on heat flux, and a forced convective region where the heat transfer depends only on the refrigerant flow rate.     

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

Heat transfer and associated frictional pressure drop in the condensing flow of the ozone friendly refrigerant R-410A in a vertical plate heat exchanger (PHE) are investigated experimentally in the present 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°. Downflow of the condensing refrigerant R-410A in one channel releases heat to the upflow of cold water in the other channel. The effects of the refrigerant mass flux, imposed heat flux, system pressure (saturated temperature) and mean vapor quality of R-410A on the measured data are explored in detail. The results indicate that the R-410A condensation heat transfer coefficient and associated frictional pressure drop in the PHE increase almost linearly with the mean vapor quality, but the system pressure only exhibits rather slight effects. Furthermore, increases in the refrigerant mass flux and imposed heat flux result in better condensation heat transfer accompanying with a larger frictional pressure drop. Besides, the imposed heat flux exhibits stronger effects on the heat transfer coefficient and pressure drop than the refrigerant mass flux especially at low refrigerant vapor quality. The friction factor is found to be strongly influenced by the refrigerant mass flux and vapor quality, but is almost independent of the imposed heat flux and saturated pressure. Finally, an empirical correlation for the R-410A condensation heat transfer coefficient in the PHE is proposed. In addition, results for the friction factor are correlated against the Boiling number and equivalent Reynolds number of the two-phase condensing flow.     

Wall boiling in a vertical annulus: Effect of inlet subcooling and mass flow rate

Numerical studies on low flow rate convection boiling in a vertical annulus has been carried out to predict effects of inlet subcooling and mass flow rate. The aspect ratio of vertical annulus is 352 while the annular gap is 3.5?mm. RPI wall boiling model is used for the development of present code and the results are verified with those available in literature. The results show that onset of significant void (OSV) can be delayed to achieve maximum heat transfer by increasing the liquid subcooling and liquid mass flow rate. The average Nusselt number increases almost linearly with increase in the mass flow rate as well as the inlet subcooling. At high heat flux, very high wall temperatures are observed with low subcooling and low mass flow rates. This should be avoided for enhanced safety.     

Two-Phase Flow Visualization in Chevron and Bumpy Style Flat Plate Heat Exchangers

Adiabatic flow visualization in a chevron plate, a 1:1 aspect ratio bumpy plate, and a 2:1 aspect ratio bumpy plate heat exchangers were investigated for vertical upward flow with R134a. Qualities ranging from 5% to 90% and mass fluxes of 60, 90, and 125 kg/m²-s were investigated. The flow visualization experiments were conducted at a 10°C inlet temperature. Four flow regimes were observed for the flat plate geometries investigated: bubbly flow, rough annular flow, smooth annular flow, and mist flow. The four flow regimes are mapped out on a mass flux versus quality basis for each geometry. The chevron geometry was seen to undergo flow transitions at lower qualities and mass fluxes than the bumpy plate geometries, and the 2:1 aspect ratio bumpy plate geometry was seen to undergo flow transitions at lower qualities and mass fluxes than the 1:1 aspect ratio bumpy plate geometry.     

水平方形管进口区域单面冷却凝结换热

对方形管进口区蒸汽单侧冷却凝结进行可视化观测及参数测量，发现随蒸汽雷诺数(Re为1669～5553)的提高，凝结液成膜方式、发展演化和稳定性均与低雷诺数下由液滴、液桥合并形成的稳定液膜有较大差异。液膜不同流动形态，如周期性断裂、局部失稳、小溪流，对换热的影响十分显，进口区域存在的高换热特性正是由于液膜流动方式的不稳定性所致。高蒸汽雷诺数(Re为5553)时，蒸汽流动的脉动性、界面切应力及Marangoni效应是导致液膜断裂的主要原因。     

Visualization study of steam condensation in triangular microchannels

A visualization experiment is conducted to investigate the condensation of steam in a series of triangular silicon microchannels. The results indicate that droplet, annular, injection and slug-bubbly flow are the dominant flow patterns in these triangular silicon microchannels. With increased mass flow rate, or an increase in the hydraulic diameter under the same Reynolds number, the location at which the injection occurred is observed to move towards the channel outlet. The frequency of the injection increases, i.e. the flow of condensation instability is higher with increased inlet vapor Reynolds number, condensate Weber number and the prolongation of the injection location, or with a decrease in the hydraulic diameter of the channel. In addition, the wall temperature of the channel decreases along the condensation stream. The total pressure drop, the average condensation heat transfer coefficient and the average Nusselt number are observed to be larger with increased inlet vapor Reynolds number. Moreover, it is found that the condensation heat transfer is enhanced by a reduction in the channel scale.     

Microchannel size effects on local flow boiling heat transfer to a dielectric fluid

Heat transfer with liquid–vapor phase change in microchannels can support very high heat fluxes for use in applications such as the thermal management of high-performance electronics. However, the effects of channel cross-sectional dimensions on the two-phase heat transfer coefficient and pressure drop have not been investigated extensively. In the present work, experiments are conducted to investigate the local flow boiling heat transfer of a dielectric fluid, Fluorinert FC-77, in microchannel heat sinks. Experiments are performed for mass fluxes ranging from 250 to 1600 kg/m² s. Seven different test pieces made from silicon and consisting of parallel microchannels with nominal widths ranging from 100 to 5850 μm, all with a nominal depth of 400 μm, are considered. An array of temperature sensors on the substrate allows for resolution of local temperatures and heat transfer coefficients. The results of this study show that for microchannels of width 400 μm and greater, the heat transfer coefficients corresponding to a fixed wall heat flux as well as the boiling curves are independent of channel size. Also, heat transfer coefficients and boiling curves are independent of mass flux in the nucleate boiling region for a fixed channel size, but are affected by mass flux as convective boiling dominates. A strong dependence of pressure drop on both channel size and mass flux is observed. The experimental results are compared to predictions from a number of existing correlations for both pool boiling and flow boiling heat transfer.     

不可凝气体对大扁管空冷凝汽器性能的影响分析

为了研究不可凝气体(non-condensable gases, NCG)对火电与光热发电机组上广泛使用的大扁管空冷凝汽器性能的影响,以工程机组凝汽器上普遍应用的通流面积220 mm×20 mm的大扁管为研究对象,针对汽轮机典型工况下的实际蒸汽流量,基于Lee相变方程、VOF方法以及组分扩散模型,对蒸汽与NCG混合气体管内两相流凝结换热进行数学建模与数值计算。结果表明：由于大扁管的狭窄通流几何结构与高蒸汽流量,NCG对管内蒸汽凝结的抑制效果要远低于预期;当入口空气质量分数按2%增加时,凝结管凝结换热系数仅下降2%左右,这与NCG导致低流量圆管凝结性能急剧下降的结论不同;空气正常泄漏不会导致空冷凝汽器性能下降而影响发电机组效率。