Measurement and prediction of heat transfer coefficient on ammonia flow boiling in a microfin plate evaporator

Thermal characteristics of ammonia flow boiling in a microfin plate evaporator are experimentally investigated. Titanium microfin heat transfer surface is manufactured to enhance boiling heat transfer. Longitudinally- and laterally-microfined surfaces are used and those performances are compared. Heat transfer coefficient of microfin plate evaporator is also compared with that of plain-surface plate evaporator. The effects of mass flux, heat flux, channel height, and saturation pressure on heat transfer coefficient are presented and discussed. The experiments are conducted for the range of mass flux (5 and 7.5 kg m⁻² s⁻¹), heat flux (10, 15, and 20 kW m⁻²), channel height (1, 2, and 5 mm), and saturation pressure (0.7 and 0.9 MPa). Heat transfer coefficient is compared with that predicted by available empirical correlations proposed by other researchers. Modified correlations using Lockhart-Martinelli parameter to predict heat transfer coefficient are developed and they cover more than 87% of the experimental data.     

An experimental investigation and modelling of flow boiling heat transfer of isobutane-compressor oil solution in a horizontal smooth tube

Experimental results of local heat transfer coefficients for the boiling of working fluids (solutions of R600a with mineral naphthenic oil ISO VG 15) in a smooth tube with a small diameter (5.4 mm) are presented. The experiments have been performed in the following ranges: for the inlet pressure from 65.7 kPa to 82.2 kPa, for the heat flux from 2500 to 3300 W m⁻², and for the mass velocity of the working fluid from 11.90 to 15.99 kg m⁻² s⁻¹). The quantitative estimation in reduction of the heat transfer coefficient of the wetted surface in the evaporator at a high oil concentration in the mixture is examined. The influence of heat flux and mass velocities on the values of the local heat transfer coefficients is analyzed. The equation for the modelling of the local heat transfer coefficient for boiling of an isobutane/compressor oil solution flow in the tube is suggested.     

Evaporation heat transfer and pressure drop of ammonia in a mixed configuration chevron plate heat exchanger

Ammonia is a naturally occurring environment friendly refrigerant with attractive thermo-physical properties. Experimental investigation of heat transfer and pressure drop during steady state evaporation of ammonia in a commercial plate heat exchanger has been carried out for an un-symmetric 30°/60° chevron plate configuration. Experiments were conducted for saturation temperatures ranging from −25 °C to −2 °C. The heat flux was varied between 21 kW m⁻² and 44 kW m⁻². Experimental results show significant effect of saturation temperature, heat flux and exit vapor quality on heat transfer coefficient and pressure drop. Current mixed plate configuration data are compared with previous studies on the same heat exchanger with symmetric plate configurations. This comparison highlighted importance of optimization in selection of the heat exchangers. Correlations for two phase Nusselt number and friction factor for each chevron plate configuration considered are developed. A Nusselt number correlation generalized for a range of chevron angles is also proposed.     

Ammonia evaporation in a mixed configuration chevron plate heat exchanger with and without miscible oil

An experimental study was conducted to determine the effects of miscible lubricant oil on evaporation of ammonia in a vertical chevron plate heat exchanger. The heat exchanger was configured in a U-type counter flow arrangement with mixed (30°/60°) chevron plate configuration. Experiments were carried out for four saturation temperatures ranging from −25 °C to −2 °C for a fixed ammonia mass flux rate of 6.5 kg m⁻² s⁻¹ and over a range of heat flux levels resulting in a vapor quality at the heat exchanger exit ranging between 0.5 and 0.9. For a given saturation temperature, experiments were performed for 0%, 3%, 6% and 9% oil concentrations, by volume in ammonia. The oil concentration, exit vapor quality, heat flux and saturation temperature were found to have significant effects on the heat transfer coefficient and pressure drop of ammonia. Based on the experimental data, correlations to estimate two phase Nusselt number and friction factor, generalized for the whole range of oil concentration have been presented.     

Condensation heat transfer and pressure drop in flattened microfin tubes having different aspect ratios

In this study, condensation heat transfer coefficients and pressure drops of R-410A are obtained in flattened microfin tubes made from 7.0 mm O.D. round microfin tubes. The test range covers saturation temperature 45 °C, mass flux 100–400 kg m⁻² s⁻¹ and quality 0.2–0.8. Results show that the effect of aspect ratio on condensation heat transfer coefficient is dependent on the flow pattern. For annular flow, the heat transfer coefficient increases as aspect ratio increases. For stratified flow, however, the heat transfer coefficient decreases as aspect ratio increases. The pressure drop always increases as aspect ratio increases. Possible reasoning is provided based on the estimated flow pattern in flat microfin tubes. Comparison with existing round microfin tube correlations is made.     

Experimental study on cooling performance of air conditioning system with dual independent evaporative condenser

Evaporative condenser is an energy efficient and environmentally friendly air conditioning equipment. This paper proposed an air conditioning system using dual independent evaporative condenser and investigated the cooling performance. Many factors, such as evaporator water inlet temperature, compressor frequency, air dry-bulb temperature, air velocity and water spray rate, which influenced the cooling performances of air conditioning system with evaporative condenser have been investigated. The results indicated that cooling capacity and coefficient of performance (COP) increased significantly with the increasing of evaporator water inlet temperature (12–25 °C), the air velocity (2.05–3.97 m s⁻¹) and the water spray rate (0.03–0.05 kg m⁻¹ s). However, COP decreased with the increasing ambient air dry-bulb temperature (31.2–35.1 °C) and the compressor frequency (50–90 Hz). Furthermore, the heat transfer coefficient (K₀) was 232–409 W m⁻² K⁻¹ in different air velocity and water spray rate.     

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.     

Experimental investigation of two phases evaporative heat transfer coefficient of carbon dioxide as a pure refrigerant and oil contaminated under forced flow conditions in small and large tube

The evaporative two-phase heat transfer coefficient of CO₂/oil contaminated as a refrigerant under forced flow conditions through a smooth horizontal tube was experimentally investigated. The experiments were carried out for two test sections of evaporators. The test sections were made of seamless precision steel tubes with a length of 1.12 m and two inner diameters of 4 and 10 mm to fulfill the influence of the evaporator geometry. Experimental parameters include mass fluxes varied from 90 to 750 (kg m⁻² s), heat flux ranged from 5 to 40 (kW m⁻²), evaporation temperatures changed from −10 to −35 °C, and the oil concentration is varied from 0.2 to 7 %. The results from the experiment are compared with those calculated from correlations reported in the literature. The results of this study are of technological importance for the efficient design of evaporators when systems are assigned to utilize CO₂ as a refrigerant.     

Refrigerant R134a condensation heat transfer and pressure drop inside a small brazed plate heat exchanger

This paper presents the experimental tests on HFC-134a condensation inside a small brazed plate heat exchanger: the effects of refrigerant mass flux, saturation temperature and vapour super-heating are investigated.A transition point between gravity controlled and forced convection condensation has been found for a refrigerant mass flux around 20 kg/m² s. For refrigerant mass flux lower than 20 kg/m² s, the saturated vapour heat transfer coefficients are not dependent on mass flux and are well predicted by the Nusselt [Nusselt, W., 1916. Die oberflachenkondensation des wasserdampfes. Z. Ver. Dt. Ing. 60, 541–546, 569–575] analysis for vertical surface. For refrigerant mass flux higher than 20 kg/m² s, the saturated vapour heat transfer coefficients depend on mass flux and are well predicted by the Akers et al. [Akers, W.W., Deans, H.A., Crosser, O.K., 1959. Condensing heat transfer within horizontal tubes. Chem. Eng. Prog. Symp. Ser. 55, 171–176] equation. In the forced convection condensation region, the heat transfer coefficients show a 30% increase for a doubling of the refrigerant mass flux. The condensation heat transfer coefficients of super-heated vapour are 8–10% higher than those of saturated vapour and are well predicted by the Webb [Webb, R.L., 1998. Convective condensation of superheated vapour. ASME J. Heat Transfer 120, 418–421] model. The heat transfer coefficients show weak sensitivity to saturation temperature. The frictional pressure drop shows a linear dependence on the kinetic energy per unit volume of the refrigerant flow and therefore a quadratic dependence on the refrigerant mass flux.     

Flow boiling of non-azeotropic mixture R32/R1234ze(E) in horizontal microfin tubes

Flow boiling of a potential refrigerant R32/R1234ze(E) in a horizontal microfin tube of 5.21 mm inner diameter is experimentally investigated. The heat transfer coefficient (HTC) and pressure drop are measured at a saturation temperature of 10 °C, heat fluxes of 10 and 15 kW m^?2, and mass velocities from 150 to 400 kg m^?2 s^?1. The HTC of R1234ze(E) is lower than that of R32. Degradation in the HTC of the R32/R1234ze(E) mixture is significant; the HTC is even lower than that of R1234ze(E). The HTC is minimized at the composition 0.2/0.8 by mass, where the temperature glide and the mass fraction distribution are maximized. A predicting correlation based on Momoki et al. (1995) associated with the correction methods of Thome (1981) to consider the mass transfer resistance and Stephan (1992) to consider the additionally required sensible heat is proposed and validated with the experimental results.     

R1234ze(E) flow boiling inside a 3.4 mm ID microfin tube

R1234ze(E) has a GWP<1 and a normal boiling temperature approximately 7.3 °C lower than that of R134a; it represents an interesting candidate for its replacement as working fluid in refrigerating machines. The refrigerant charge minimization in refrigerating and air conditioning equipment is a key issue for the new environmental challenges. Mini microfin tubes represent an optimal solution for both heat transfer enhancement and charge minimization tasks. This paper presents an experimental study of R1234ze(E) flow boiling inside a mini microfin tube with internal diameter at the fin tip of 3.4 mm. The experimental measurements were carried out at constant saturation temperature of 30 °C, by varying the refrigerant mass velocity between 190 kg m⁻² s⁻¹ and 940 kg m⁻² s⁻¹, the vapour quality from 0.2 to 0.99 at three different heat fluxes: 10, 25, and 50 kW m⁻². The experimental results are then compared with those obtained for the more traditional R134a.     

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%.     

Two-phase flow heat transfer of CO2 vaporization in smooth horizontal minichannels

Experiments were performed on the convective boiling heat transfer in horizontal minichannels with CO₂. The test section is made of stainless steel tubes with inner diameters of 1.5 and 3.0 mm and with lengths of 2000 and 3000 mm, respectively, and it is uniformly heated by applying an electric current directly to the tubes. Local heat transfer coefficients were obtained for a heat flux range of 20–40 kW m⁻², a mass flux range of 200–600 kg m⁻² s⁻¹, saturation temperatures of 10, 0, −5, and −10 °C and quality ranges of up to 1.0. Nucleate boiling heat transfer contribution was predominant, especially at low quality region. The reduction of heat transfer coefficient occurred at a lower vapor quality with a rise of heat flux, mass flux and saturation temperature, and with a smaller inner tube diameter. The experimental heat transfer coefficient of CO₂ is about three times higher than that of R-134a. Laminar flow appears in the minichannel flows. A new boiling heat transfer coefficient correlation that is based on the superposition model for CO₂ was developed with 8.41% mean deviation.     

Two-phase heat transfer and pressure drop of LNG during saturated flow boiling in a horizontal tube

Two-phase heat transfer and pressure drop of LNG (liquefied natural gas) have been measured in a horizontal smooth tube with an inner diameter of 8 mm. The experiments were conducted at inlet pressures from 0.3 to 0.7 MPa with a heat flux of 8–36 kW m⁻², and mass flux of 49.2–201.8 kg m⁻² s⁻¹. The effect of vapor quality, inlet pressure, heat flux and mass flux on the heat transfer characteristic are discussed. The comparisons of the experimental data with the predicted value by existing correlations are analyzed. Zou et al. (2010) correlation shows the best accuracy with 24.1% RMS deviation among them. Moreover four frictional pressure drop methods are also chosen to compare with the experimental database.     

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.     

Flow boiling data and prediction method for enhanced boiling tubes and tube bundles with R-134a and R-236fa including a comparison with falling film evaporation

New enhanced boiling tubes from Wolverine Tube Inc. (Turbo-B5) and Wieland-Werke AG (Gewa-B5) were investigated using R-134a and R-236fa as test fluids. The tests were done at saturation temperatures of 5 and 15 °C, mass flow rates from 4 to 35 kg m⁻² s⁻¹ and heat fluxes from 15 to 70 kW m⁻². A new prediction method based on a theoretical analysis of thin film evaporation was used to propose a new correlating parameter. A large new database of local heat transfer coefficients was obtained and utilized to generate an improved prediction method for bundle boiling and the onset of dryout. Onset of dryout and the simultaneous reduction in heat transfer performance occurred at very high vapour quality on these enhanced tubes in convective bundle boiling. Furthermore, a direct comparison was made between the tubes operating in falling film and convective bundle boiling modes.     

Nucleate boiling heat transfer coefficients of HFO1234yf on various enhanced surfaces

In this study, nucleate boiling heat transfer coefficients (HTCs) of HFO1234yf HFC134a are measured on a flat plain, Turbo-B, Turbo-C, and Thermoexcel-E surfaces. All data are taken at the liquid pool temperature of 7 °C on small flat horizontal square copper plates (9.53 mm × 9.53 mm) at heat fluxes from 10 kW m⁻² to 200 kW m⁻² with an interval of 10 kW m⁻². Test results show that nucleate boiling HTCs of HFO1234yf on all four surfaces are similar to those of HFC134a at all heat fluxes tested in this study. At heat fluxes below 150 kW m⁻², Thermoexcel-E surface shows the highest heat transfer performance and hence is the best surface for the manufacture of the evaporators in refrigeration and air-conditioning equipment. On the other hand, at high heat fluxes above 150 kW m⁻², Turbo-B and Turbo-C show better heat transfer performance than Thermoexcel-E and hence are good for electronic cooling applications. Overall, HFO1234yf is a good long term candidate with excellent environmental properties to replace successfully HFC134a from the view point of pool boiling heat transfer. Hence HFO1234yf can be readily applied to the conventional evaporators designed for HFC134a.     

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.     

CO2 flow condensation heat transfer and pressure drop in multi-port microchannels at low temperatures

CO₂ flow condensation heat transfer coefficients and pressure drop are investigated for 0.89 mm microchannels at horizontal flow conditions. They were measured at saturation temperatures of −15 and −25 °C, mass fluxes from 200 to 800 kg m⁻² s⁻¹, and wall subcooling temperatures from 2 to 4 °C. Flow patterns for experimental conditions were predicted by two flow pattern maps, and it could be predicted that annular flow patterns could exist in most of flow conditions except low mass flux and low vapor quality conditions. Measured heat transfer coefficients increased with the increase of mass fluxes and vapor qualities, whereas they were almost independent of wall subcooling temperature changes. Several correlations could predict heat transfer coefficients within acceptable error range, and from this comparison, it could be inferred that the flow condensation mechanism in 0.89 mm channels should be similar to that in large tubes. CO₂ two-phase pressure drop, measured in adiabatic conditions, increased with the increase of mass flux and vapor quality, and it decreased with the increase of saturation temperature. By comparing measured pressure drop with calculated values, it was shown that several correlations could predict the measured values relatively well.     

Convective boiling performance of refrigerant R-134a in herringbone and microfin copper tubes

This paper reports an experimental investigation of convective boiling heat transfer and pressure drop of refrigerant R-134a in smooth, standard microfin and herringbone copper tubes of 9.52 mm external diameter. Tests have been conducted under the following conditions: inlet saturation temperature of 5 °C, qualities from 5 to 90%, mass velocity from 100 to 500 kg s⁻¹ m⁻², and a heat flux of 5 kW m⁻². Experimental results indicate that the herringbone tube has a distinct heat transfer performance over the mass velocity range considered in the present study. Thermal performance of the herringbone tube has been found better than that of the standard microfin in the high range of mass velocities, and worst for the smallest mass velocity (G=100 kg s⁻¹ m⁻²) at qualities higher than 50%. The herringbone tube pressure drop is higher than that of the standard microfin tube over the whole range of mass velocities and qualities. The enhancement parameter is higher than one for both tubes for mass velocities lower than 200 kg s⁻¹ m⁻². Values lower than one have been obtained for both tubes in the mass velocity upper range as a result of a significant pressure drop increment not followed by a correspondent increment in the heat transfer coefficient.