Two-phase heat transfer to a refrigerant in a 1 mm diameter tubeTransfert de chaleur vers un frigorigène circulant dans un tube d'un diamètre d'un mm

A study of two-phase flow and heat transfer in a small tube of 1 mm internal diameter has been conducted experimentally as part of a wider study of boiling in small channels. R141b has been used as the working fluid. The boiling heat transfer in the small tube has been measured over a mass flux range of 300–2000 kg/m² s and heat flux range of 10–1150 kW/m². In this paper the boiling map for a mass velocity of 510 kg/m² s and heat flux of 18–72 kW/m² is discussed and the problems of determining heat transfer coefficients in small channels are highlighted.     

Unified correlation for heat transfer during boiling in plain mini/micro and conventional channels

A correlation is presented for predicting heat transfer coefficients during saturated boiling prior to critical heat flux in mini/micro channels as well as channels of conventional sizes in horizontal and vertical upward flow. The correlation is verified with a database that includes channels of various shapes (round, rectangle, triangle), fully or partially heated, horizontal and vertical downflow, diameters 0.38 to 27.1 mm, 30 fluids (water, CO₂, ammonia, halocarbon refrigerants, organics, cryogens), reduced pressure 0.0046 to 0.787, and mass flux 15 to 2437 kg m⁻²s⁻¹. The new correlation predicts the 4852 data points from 137 data sets from 81 sources with a mean absolute deviation of 18.6 %. Several other correlations were also compared with the same database; all had significantly higher deviations.     

Characterisation of the hydraulic maldistribution in a heat exchanger by local measurement of convective heat transfer coefficients using infrared thermography

A methodology was developed to characterise the heat exchangers' performance decrease due to two-phase flow maldistribution. It consists in measuring the spatial distribution of the local heat transfer coefficients with a rapid, non-invasive and fluid independent method. The method is based on the infrared (IR) thermography measurement of the temperature response to an oscillating heat flux. The amplitude of the measured temperatures is compared to the solution of an analytical model. The problem is solved iteratively to obtain the heat transfer coefficients. This method has been applied to evaluate the uneven phase distribution of an air–water mixture in a compact heat exchanger. The exchanger is composed of seven multiport flat tubes, a vertical downward header and horizontal channels. Experiments were performed for mass flux from 29 kg m⁻² s⁻¹ to 116 kg m⁻² s⁻¹ and for quality from 0.10 to 0.70.     

Boiling flow through diverging microchannel

An experimental study of flow boiling through diverging microchannel has been carried out in this work, with the aim of understanding boiling in non-uniform cross-section microchannel. Diverging microchannel of 4° of divergence angle and 146 μm hydraulic diameter (calculated at mid-length) has been employed for the present study with deionised water as working fluid. Effect of mass flux (118–1182 kg/m²-s) and heat flux (1.6–19.2 W/cm²) on single and two-phase pressure drop and average heat transfer coefficient has been studied. Concurrently, flow visualization is carried out to document the various flow regimes and to correlate the pressure drop and average heat transfer coefficient to the underlying flow regime. Four flow regimes have been identified from the measurements: bubbly, slug, slug–annular and periodic dry-out/ rewetting. Variation of pressure drop with heat flux shows one maxima which corresponds to transition from bubbly to slug flow. It is shown that significantly large heat transfer coefficient (up to 107 kW/m²-K) can be attained for such systems, for small pressure drop penalty and with good flow stability.     

Flow Boiling Heat Transfer in Two-Phase Micro Channel Heat Sink at Low Water Mass Flux

Boiling heat transfer at water flow with low mass flux in heat sink which contained rectangular microchannels was studied. The stainless steel heat sink contained ten parallel microchannels with a size of 640 × 2050 μm in cross-section with typical wall roughness of 10–15 μm. The local flow boiling heat transfer coefficients were measured at mass velocity of 17 and 51 kg/m²s, heat flux on 30 to 150 kW/m² and vapor quality of up to 0.8 at pressure in the channels closed to atmospheric one. It was observed that Kandlikar nucleate boiling correlation is in good agreement with the experimental data at mass flow velocity of 85 kg/m²s. At smaller mass flux the Kandlikar model and Zhang, Hibiki and Mishima model demonstrate incorrect trend of heat transfer coefficients variation with vapor quality.     

Condensation of refrigerants in a multiport tube with rectangular minichannels

This study investigated the condensation heat transfer and pressure drop characteristics of refrigerants R134a, R32, R1234ze(E), and R410A in a horizontal multiport tube with rectangular minichannels, in the mass velocity range of 100–400 kg m⁻² s⁻¹ and saturation temperature set at 40 and 60 °C. The effect of mass velocity, vapor quality, saturation temperature, refrigerant properties, and hydraulic diameter of rectangular channels on condensation characteristics is clarified. A new correlation is proposed for predicting the frictional pressure drop for condensation flow in minichannels. A heat transfer model for condensation heat transfer in rectangular minichannels is developed considering the flow patterns and effects of vapor shear stress and surface tension. Then, based on this model, a new heat transfer correlation is proposed. The proposed correlations successfully predict the experimental frictional pressure drop and heat transfer coefficients of the test refrigerants in horizontal rectangular minichannels.     

Heat transfer of oil-contaminated HFC134a in a horizontal evaporator

The introduction of chlorine-free refrigerants to the market requires experimental investigations of their behaviour in heat pumps and refrigerators. One particular area of interest is the effect of the new oils on the heat transfer in evaporators and condensers. Oil can either increase or decrease the heat transfer coefficient. This paper presents the results from an experimental investigation of the effect of three different ester-based oils on the heat transfer of HFC134a in a horizontal evaporator. The tests were carried out at heat fluxes between 2 and 8 kW m⁻² (corresponding to mass fluxes between approximately 40 and 170 kg s⁻¹ m⁻²). The evaporation temperature was varied from−10 to +10°C. The global oil concentration ranged from 0 to 4.5 mass percentage based on the total liquid flow. The heat transfer coefficient decreased in most of the cases. The results indicate that the decrease seems to depend on the viscosity of the oil. The decrease can fairly well be estimated with the correlation for pure refrigerants by Shah if the viscosity of the mixture is used in the calculations. The data for the oil-contaminated refrigerant also agree well with data for pure refrigerants in a plot of α_tp/α_lo^* versus the inverse Martinelli-Lockhart parameter when α_lo^* is calculated with a modified Dittus-Boelter correlation and the mixture viscosity is used in the calculations. The heat transfer is found to increase when introducing oil in the special cases where the flow rate is low and the viscosity is low (oil A, 2 and 4 kW m⁻² oil B, 6kW m⁻² at +10°C). This is most likely due to surface tension effects. It has been suggested that the increased surface tension leads to a better tube wetting and thus an increased heat transfer.     

Investigation of forced flow boiling of nitrogen in a long vertical tube

The results of experimental investigations of nitrogen boiling in a vertical tube 1850 mm long id 10 mm, are presented. It has been established that compared with pool boiling, heat flux and pressure effects are somewhat less pronounced and the boiling curve hysteresis phenomenon is missing. The transition from the subcooled boiling region to saturated boiling is shown not to be accompanied by a change of heat transfer intensity. Experimental data obtained over the entire range of parameters (p = 1.8–7.0 bar, G = 170–750 kg m^?2 s^?1, q = 10 200-42 720 W m^?2) are correlated by an equation.     

Forced convection heat transfer to liquid helium I in the nucleate boiling region

Heat transfer and critical heat fluxes to helium boiling in a 2 mm id copper tube (100 mm long) were measured in the pressure range 1.1–1.5 atm and at mass velocities 18–96 kg m^?2s^?1. Corresponding Reynolds numbers are (1.2–6.2) × 10⁴. Experimentally obtained heat transfer coefficients show satisfactory agreement with those calculated according to the Kutateladze equation but with less pronounced pressure dependence. It was found that in the boiling region developed quality did not influence the heat transfer coefficient. An expression was obtained, which describes with ±10% error, the dependence of critical heat flux on mass flow rate in the pressure range 1.1–1.5 atm and mass quality 0.33–0.6.     

Flow boiling heat transfer to carbon dioxide: general prediction method

An updated version of the Kattan–Thome–Favrat flow pattern based, flow boiling heat transfer model for horizontal tubes has been developed specifically for CO₂. Because CO₂ has a low critical temperature and hence high evaporating pressures compared to our previous database, it was found necessary to first correct the nucleate pool boiling correlation to better describe CO₂ at high reduced pressures and secondly to include a boiling suppression factor on the nucleate boiling heat transfer coefficient to capture the trends in the flow boiling data. The new method predicts 73% of the CO₂ database (404 data points) to within ±20% and 86% to within ±30% over the vapor quality range of 2–91%. The database covers five tube diameters from 0.79 to 10.06 mm, mass velocities from 85 to 1440 kg m⁻² s⁻¹, heat fluxes from 5 to 36 kW m⁻², saturation temperatures from −25 °C to +25 °C and saturation pressures from 1.7 to 6.4 MPa (reduced pressures up to 0.87).     

An experimental investigation of saturated flow boiling heat transfer and pressure drop in square microchannels

In this study, saturated flow boiling characteristics of deionized water in parallel microchannels are investigated experimentally. The silicone microchannel heat sink consists of 29 parallel square microchannels having hydraulic diameters of 150 µm. Experiments have been conducted for four different values of the mass flux consisting of 51, 64.5, 78 and 92.6 kg/m²s and heat flux values from 59.3 to 84.1 kW/m². Inlet temperature of deionized water is kept at 50 ± 1 °C. Heat transfer and pressure drop are examined for varying values of the governing parameters. Simultaneous high-speed video images have been taken as well as temperature and pressure measurements. The flow visualization results lead to key findings for flow boiling instabilities and underlying physical mechanisms of heat transfer in microchannels. Quasi-periodical rewetting and drying, rapid bubble growth and elongation toward both upstream and downstream of the channels and reverse flow are observed in parallel microchannels.     

Experimental investigation on flow condensation heat transfer and pressure drop of R170 in a horizontal tube

Condensation heat transfer and pressure drop of R170 were studied experimentally in a horizontal tube with inner diameter of 4 mm. The tests were conducted at saturation pressures from 1 MPa to 2.5 MPa, mass fluxes from 100 kg (m²∙s)⁻¹ to 250 kg (m²∙s)⁻¹ and average heat fluxes from 55.3 kW m⁻² to 96.3 kW m⁻² over the entire vapor quality range. The effects of vapor quality, mass flux and saturation pressure on condensation heat transfer and pressure drop were examined and analyzed. The experimental data were compared with various well-known correlations of condensation heat transfer coefficient and pressure drop. The comparison results showed that Koyama et al. correlation agreed with the experimental heat transfer coefficient with a mean absolute relative deviation less than 25%, and the Yan and Lin correlation can accurately predict the experimental pressure drop with a mean absolute relative deviation less than 18%.     

Design of a plate-type evaporator for heat pumpsConception d'un évaporateur à plaques pour pompes à chaleur

Use of particularly cold and/or polluted water as a heat source for heat pumps makes special demands on evaporator design. The plate-type evaporator is probably the most suitable design. It can be cleaned easily and ice formation does not cause any damage to it. High plate efficiency is attained by using narrow, vertical refrigerant flow channels. On the water side falling film flow is recommended. Under such conditions, evaporator k-values of up to approximately 1400 W m^?2 K^?1 and 1200 W m^?2 are reached in laboratory tests, using refrigerant R 22 and R 12 respectively. Experiments have shown that water of 2°C and even lower temperatures may be used. This opens for use significant heat sources, which probably could not be exploited economically with other evaporator types.     

Micro technology in heat pumping systems

Micro heat pumps, with dimensions in the order of centimetres, may in the future be utilised for the heating and/or cooling of buildings, vehicles, clothing, and other products or applications. A number of issues have yet to be solved, including the construction of a microscale compressor, and determination of micro heat exchanger heat transfer capacities. Test samples of micro heat exchangers and a corresponding test apparatus have been built. Some two-phase experiments with propane (R-290) as refrigerant have been conducted. Preliminary results for a micro condenser with 0.5 mm wide trapezoidal channels of 25 mm length showed that a heat flux of up to 135 kW/m², based on the refrigerant-side area, was attainable. The corresponding overall heat transfer coefficient was 10 kW/(m² K), with a refrigerant mass flux of 165 kg/(m² s) and a refrigerant-side pressure drop of 180 kPa/m.     

New experimental data of CO2 flow boiling in mini tube with micro fins of zero helix angle

Heat transfer and pressure drop characteristics of CO₂ flow boiling in mini tube with micro fins of zero helix angle were experimentally investigated. The working conditions cover mass flux from 100 to 600 kg m⁻² s⁻¹, heat flux from 1.67 to 8.33 kW m⁻², vapor quality from 0 to 0.9 and saturation temperature from 1 to 15 °C. The results show that the heat transfer coefficient increases with increasing vapor quality, but sharply decreases at vapor quality around 0.2~0.4 under most conditions, and the dryout vapor quality decreases with the increasing heat flux and saturation temperature. Pressure drop increases with increasing mass flux and heat flux, or decreasing saturation temperature, and mass flux is the major influence factors. The enhancement ratio of heat transfer coefficient is higher than that of pressure drop, which shows potentials of using such kind tubes to enhance the overall heat transfer performance. A heat transfer coefficient correlation and a pressure drop correlation for 0° helix angle micro-fin tube were developed, and they agree well with the experimental data.     

A minichannel aluminium tube heat exchanger – Part II: Evaporator performance with propane

This paper presents heat transfer data for a multiport minichannel heat exchanger vertically mounted as an evaporator in a test-rig simulating a small water-to-water heat pump. The multiport minichannel heat exchanger was designed similar to a shell-and-tube type heat exchanger, with a six-channel tube of 1.42 mm hydraulic diameter, a tube-side heat transfer area of 0.777 m² and a shell-side heat transfer area of 0.815 m². Refrigerant propane with a desired vapour quality flowed upward through the tubes and exited with a desired superheat of 1–4 K. A temperature-controlled glycol solution that flowed downward on the shell-side supplied the heat for the evaporation of the propane. The heat transfer rate between the glycol solution and propane was controlled by varying the evaporation temperature and propane mass flow rate while the glycol flow rate was fixed (18.50 l min⁻¹). Tests were conducted for a range of evaporation temperatures from −15 to +10 °C, heat flux from 2000 to 9000 W m⁻² and mass flux from 13 to 66 kg m⁻² s⁻¹. The heat transfer coefficients were compared with 14 correlations found in the literature. The experimental heat transfer coefficients were higher than those predicted by many of the correlations. A correlation which was previously developed for a very large and long tube (21 mm diameter and 10 m long) was in good agreement with the experimental data (97% of the data within ±30%). Several other correlations were able to predict the data within a reasonable deviation (within ±30%) after some adjustments to the correlations.     

A minichannel aluminium tube heat exchanger – Part I: Evaluation of single-phase heat transfer coefficients by the Wilson plot method

A prototype liquid-to-refrigerant heat exchanger was developed with the aim of minimizing the refrigerant charge in small systems. To allow correct calculation of the refrigerant side heat transfer, the heat exchanger was first tested for liquid-to-liquid (water-to-water) operation in order to determine the single-phase heat transfer performance. These single-phase tests are reported in this paper. The heat exchanger was made from extruded multiport aluminium tubes and was designed similar to a shell-and-tube heat exchanger. The heat transfer areas of the shell-side and tube-side were approximately 0.82 m² and 0.78 m², respectively. There were six rectangular-shaped parallel channels in a tube. The hydraulic diameter of the tube-side was 1.42 mm and of the shell-side 3.62 mm. Tests were conducted with varying water flow rates, temperature levels and heat fluxes on both the tube and shell sides at Reynolds numbers of approximately 170–6000 on the tube-side and 1000–5000 on the shell-side, respectively. The Wilson plot method was employed to investigate the heat transfer on both the shell and tube sides. In the Reynolds number range of 2300–6000, it was found that the Nusselt numbers agreed with those predicted by the Gnielinski correlation within ±5% accuracy. In the Reynolds number range of 170–1200 the Nusselt numbers gradually increased from 2.1 to 3.7. None of the previously reported correlations for laminar flow predicted the Nusselt numbers well in this range. The shell-side Nusselt numbers were found to be considerably higher than those predicted by correlations from the literature.     

Effects of coolant on temperature distribution in metal machining

Abstract

The effects of coolant on tool temperature in metal turning operations were studied both experimentally and theoretically. Values for the coefficient of heat transfer from the tool to the coolant were obtained. Experimentally, a medium carbon steel was turned at a feed rate of 0·254 mm rev ^?1, at cutting speeds of 33–61 m min^?1, with high speed steel tools and water based coolant. Tool temperatures estimated from structural changes in the tool were compared with values calculated by finite element analysis. Heat transfer coefficients from tool to coolant between 10³ and 5 × 10³ W m²K^?1, depending on coolant flow rate, best match theory and experiment. Further, it is shown by the calculations that tool temperatures are particularly sensitive to changes in heat transfer coefficient between 10³ and 10⁴ W m^?2K^?1. This is the range in which coefficients are to be found in practice, explaining the critical nature of cutting fluid formulation, supply rate, and direction of application in those operations where cooling is important.

MST/809     

Experimental Investigations on Condensation in the Framework of ENhanced COndensers in Microgravity (ENCOM-2) Project

We report preliminary results on experimental investigations on condensation in the framework of the European Space Agency funded programme Enhanced Condensers in Microgravity (ENCOM-2) which aims at better understanding underlying phenomena during condensation. The first experiment is a study on condensation of HFE on external curvilinear surface of 15 mm height during reduced gravity experiments. It is found that the local minimum of the film thickness exists at the conjugation area of condensed film and the meniscus at the bottom of the fin; this leads to the local maximum of the heat transfer coefficient, which we also found moves towards the fin tip. The second experiment is a study of falling films hydrodynamics inside a vertical long pipe. In particular, characteristics of wavy falling films produced employing intermittent liquid feed are examined in order to assess wave effects on film condensation. Preliminary results suggest that intermittent feed simply divides the film in two autonomous regions with the wave feature of each one depending only on its flow rate. The processing of registered film thickness data can lead to the estimation of the transverse velocity profile in the film, which is mainly responsible for heat transfer during condensation. The third experiment looks at in-tube convective condensation at low mass fluxes (typical of Loop Heat Pipes and Capillary Pumped Loops) of n-pentane inside a 0.56 mm diameter channel. The results show that the mean heat transfer in the annular zone when it is elongated may be less than the mean heat transfer when it is shorter, due to the interface deformation involved by surface tension effect. When the length of this annular zone reaches a critical value, the interface becomes unstable, and a liquid bridge forms, involving the release of a bubble. The heat transfer due to the phase-change in this isolated bubble zone appears to be very small compared to the sensible heat transfer: the bubbles evolve and collapse in a highly subcooled liquid. The last experiment concerns in-tube condensation of R134a inside a square channel of 1.23 mm hydraulic diameter at mass fluxes of 135 kg m^?2 s^?1 and 390 kg m^?2 s^?1 for three different configurations: horizontal, vertical downflow and vertical upflow. For the calculated heat transfer coefficient it is found that gravity has no effect on condensation in downflow configurations at 390 kg m^?2 s^?1 and in upflow conditions at both values of mass velocity. The effect of gravity on the condensation heat transfer coefficient becomes noteworthy in downflow at mass velocity G = 135 kg m^?2 s^?1 and vapour quality lower than 0.6.     

Non-similarity solution and correlation of transient heat transfer in laminar boundary layer flow over a wedge

With a comprehensive and rigorous method, this paper has successfully examined the transient heat transfer in a steady and two-dimensional (2D) laminar boundary layer flow on a wedge with sudden change of thermal boundary conditions of uniform wall temperature (UWT) and heat flux (UHF). Additionally, a correlation of unsteady forced convection was also formulated through an exact solution of transient heat conduction (ξ=0) and the similarity solutions of a steady forced convection on a wedge (ξ=1) in this study. Particularly, for the wedge with −0.198838?ξ?1, the deviation of the wall temperatures estimated by correlation is less than 7.5% within full time of 0?ξ?1 comparing with numerical results in the case of UHF ranging from Pr=10⁻⁴ to 10⁴.