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.     

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.     

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 pressure drop and flow boiling heat transfer in an enhanced dimpled tube with a solid round rod insert

An experimental study was conducted on a 19.05 mm (outer diameter) dimpled enhanced tube to evaluate the in-tube two phase heat transfer and pressure drop performance in an annular section created between the enhanced tube and a solid round PVC rod. The purpose of the study was to understand the effect of forced early transition to annular flow on the pressure drop and heat transfer coefficient in a horizontal tube. The refrigerant studied was R-134a at a saturation temperature of 5 °C, heat flux range 2.5 to 15 kW m⁻², mass flux from 80 to 200 kg m⁻² s⁻¹ and inlet vapor quality of 0.12 to 0.72. Flow pattern and pressure drop results were obtained under adiabatic conditions. Under similar operating conditions the enhanced tube with a rod exhibited three times higher heat transfer performance versus same size smooth empty tube with lower pressure drop penalty at lower mas flux.     

Comparison and development of new correlation for adiabatic two-phase pressure drop of refrigerant flowing inside a multiport minichannel with and without fins

This study examines the experimental adiabatic two-phase frictional pressure drop of R134a in rectangular multiport minichannel with and without fins having 20 channels with hydraulic diameters of 0.64 mm and 0.81 mm, respectively. The pressure drop measurements were done under mass flux range of 50–200 kg m⁻² s⁻¹, saturation temperature range of 20–35 °C, and inlet vapor quality range of 0.1–0.9. The effects of mass flux, saturation temperature, inlet vapor quality and channel geometry on frictional pressure drop were investigated. The results discovered that the mass flux, inlet vapor quality, saturation temperature and channel geometry play an important role in increasing or decreasing the frictional pressure drop. The present experimental data were compared with eleven existing well known frictional pressure drop correlation available in the open literature. In addition, a new two-phase frictional pressure drop correlation is proposed considering the effects of inertia, viscous force, fluid properties, channel geometry and surface tension and also validated the correlation with the available data.     

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.     

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.     

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.     

Condensing two-phase pressure drop and heat transfer coefficient of propane in a horizontal multiport mini-channel tube: Experimental measurements

This article reports the condensing flow heat transfer coefficient and pressure drop results of propane (R290) flowing through a square section horizontal multiport mini-channel tube made of aluminium having an internal diameter of 1.16 mm and a condensing length of 259 mm. Pressure drop and two phase flow experiments were performed at saturation temperatures of 30, 40 and 50 °C. Heat flux was varied from 15.76 to 32.25 kWm⁻² and mass velocity varied from 175 to 350 kg m⁻² s⁻¹. The results show that the two-phase friction pressure gradient increases with the increase of mass velocity and vapour quality and with the decrease of saturation temperature. The heat transfer coefficients showed to increase with increases of vapour quality and mass velocity while increases of saturation temperature were observed to reduce heat transfer coefficient. The two phase frictional pressure drop correlations of Sun and Mishima and Agarwal and Garimella, and the two-phase flow heat transfer correlations of Koyama et al. and Wang et al. predicted well the experimental results.     

R1234yf condensation inside a 3.4 mm ID horizontal microfin tube

This paper shows experimental results about R1234yf condensation inside a microfin tube with an inner diameter at the fin tip of 3.4 mm. R1234yf is a new environmentally friendly refrigerant, with a Global Warming Potential lower than 1, therefore it matches the new environmental laws. Experimental tests are carried out for mass velocities from 100 to 1000 kg m⁻² s⁻¹, vapor qualities from 0.95 to 0.2, at saturation temperature of 30 °C and 40 °C. The experimental results show that heat transfer coefficient increases when both mass velocity and vapor quality increase. Frictional pressure gradient increases with mass velocity at constant vapor quality, whereas at constant mass velocity it increases with vapor quality up to a maximum, after which it slightly decreases. The experimental heat transfer coefficient and pressure drop are also compared against the values predicted by empirical correlations available in the open literature.     

Condensation heat transfer and pressure drop characteristics of CO2 in a microchannel

The condensation heat transfer coefficient and pressure drop of CO₂ in a multiport microchannel with a hydraulic diameter of 1.5 mm was investigated with variation of the mass flux from 400 to 1000 kgm⁻²s⁻¹ and of the condensation temperature from −5 to 5 °C. The heat transfer coefficient and pressure drop increased with the decrease of condensation temperature and the increase of mass flux. However, the rate of increase of the heat transfer coefficient was retarded by these changes. The gradient of the pressure drop with respect to vapor quality is significant with the increase of mass flux. The existing models for heat transfer coefficient overpredicted the experimental data, and the deviation increased at high vapor quality and at high heat transfer coefficient. The smallest mean deviation of ±51.8% was found by the Thome et al. model. For the pressure drop, the Mishima and Hibiki model showed mean deviation of 29.1%.     

Experimental investigation on flow boiling pressure drop of R-290 and R-600a in a horizontal small tube

Pressure drop for propane and isobutane were performed in a horizontal small tube of stainless steel with 1.0 mm inner diameter. The tests were conducted at mass fluxes from 240 to 480 kg m⁻²s⁻¹ and heat fluxes from 5 to 60 kW m⁻² at 25 °C saturation temperature. The effect of flow patterns, mass flux, vapour quality and heat flux are discussed. Strong influence of mass flux and vapour quality on pressure drop was found. The comparisons of experimental data with predicted value proposed by existing correlations available in literature for pressure drop are analyzed.     

Review of flow condensation of CO2 as a refrigerant

Carbon dioxide (CO₂) has emerged as an excellent substitute natural refrigerant for low temperature refrigeration applications, but a better understanding of its in-tube flow condensation is needed in order to achieve its full potential. From experimental studies in the open literature we review the effects of mass flux, vapour quality and saturation pressure on CO₂ flow condensation heat transfer, frictional pressure drop and flow regime transition inside smooth, micro-fin and microchannel tubes. Successful condensation models which were developed from experiments with other refrigerants are evaluated against the CO₂ flow condensation experimental data. Comparison between the predicted and experimental data shows that the unique thermophysical properties of CO₂ at high reduced pressure conditions lead to these correlations having high prediction errors on the flow condensation heat transfer inside smooth tubes and microchannels, but have less significant effects on the flow condensation heat transfer and two-phase frictional pressure drop under high mass flux conditions inside micro-fin tubes. Recommendations for condensation and pressure drop models to apply to CO₂ flow condensation in different tubes are made. As there is inconsistency between the experimental data in smooth tubes from different sources, and the effects of microchannel and micro-fin tube geometries, on the flow regime transition and condensation heat transfer of CO₂, are unclear, a more extensive range of the experimental data in different tubes is needed for a fully understanding of in-tube CO₂ flow condensation.     

Pressure drop during flow boiling inside parallel microchannels

Pressure drop is experimentally investigated inside parallel microchannels during subcooled flow boiling of R134a in horizontal orientation. The test conditions included the inlet pressure, the inlet subcooled degree, the heat flux, the vapor quality, and the mass velocity, ranging from 600 to 900 kPa, 1 to 20 K, 5 to 220 kW m⁻², 0 to 95% and 250 to 1000 kg m⁻² s⁻¹, respectively. The effect of the mass velocity and the inlet pressure were investigated. The relative weight of the pressure drop due to two-phase flow acceleration and the friction pressure drop for single phase and two-phase flows were considered. The experimental results for the pressure drop were compared with those predicted by the homogenous model and five other semi-empirical models.     

HFC404A condensation inside a small brazed plate heat exchanger: Comparison with the low GWP substitutes propane and propylene

This paper compares HFC404A and two possible hydrocarbon substitutes HC290 (propane) and HC1270 (propylene) in condensation inside a commercial brazed plate heat exchanger (BPHE) at saturation temperatures between 25 and 40 °C. The effects of saturation temperature (pressure), refrigerant mass flux, and vapour super-heating on heat transfer and hydraulic performances are investigated. The transition between gravity dominated and forced convection condensation occurs at a mass flux around 20 kg m⁻² s⁻¹. In forced convection condensation a doubling of the refrigerant mass flux produces a 30% enhancement of the heat transfer coefficient. The condensation heat transfer coefficients of super-heated vapour are 5–10% higher than those of saturated vapour. Propane and propylene exhibit condensation heat transfer coefficients 25–45% higher and frictional pressure drops similar to those of HFC404A under comparable operating conditions. Therefore both propane and propylene are very promising as low GWP definitive substitutes for HFC404A in industrial and commercial refrigeration.     

Experimental study on thermal characteristics of ammonia flow boiling in a plate evaporator at low mass flux

Thermal characteristics of a plate evaporator using ammonia are experimentally investigated. The effects of mass flux, heat flux, channel height, and saturation pressure on heat transfer coefficient of the evaporator are discussed. The experiments are conducted for 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 obtained as a function of quality for all experimental conditions. The characteristics of heat transfer coefficient are discussed and compared with those of earlier works. All experimental results are compiled by using Lockhart–Martinelli parameter. The developed empirical correlation predicts 85% of the experimental data within ±30% range.     

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.     

Condensation of R407C in a horizontal microfin tube

This paper describes experimental results that show the effects of mass velocity and condensation temperature difference on the local heat transfer characteristics during condensation of R407C in a horizontal microfin tube. The experiments were performed at the saturation temperature of 40 °C, the refrigerant mass velocity of 50, 100, 200 and 300 kg m⁻² s⁻¹, and the condensation temperature difference of 1.5, 2.5 and 4.5 K. A superficial heat transfer coefficient for the vapor phase was obtained by subtracting the heat transfer resistance of condensate film estimated by using a previously developed theoretical model of film condensation of pure vapor from the overall heat transfer resistance. On the basis of the analogy between heat and mass transfer, an empirical equation for the superficial vapor phase heat transfer coefficient was developed. The heat transfer coefficient predicted by the combination of the previously developed theoretical model of film condensation of pure vapor and the empirical equation of the superficial vapor phase heat transfer coefficient agreed with the measured values with the r.m.s. error of 9.2%.     

Experimental investigation on heat transfer characteristics of supercritical CO2 cooled in horizontal helically coiled tube

An experimental investigation on the heat transfer characteristics of supercritical CO₂ during gas cooling process in a helically coiled tube is conducted. The experimental data are obtained over a mass flux range of 79.6–238.7 kg m⁻² s⁻¹, an inlet pressure range of 7.5–9.0 MPa and a mean bulk temperature of 23.0–53.0 °C. The effects of mass flux, bulk temperature and pressure on the heat transfer coefficient for helically coiled tubes are investigated. A comparative analysis of the gravitational buoyancy and the heat transfer coefficient is carried out between helically coiled tubes and straight tubes. A new heat transfer correlation of the supercritical CO₂ in the horizontal helically coiled tube is proposed based on the experimental data. The maximum error between the predicted results of the new correlation and the experimental data is 20%.     

Effect of tube diameter on boiling heat transfer and flow characteristic of refrigerant R32 in horizontal small-diameter tubes

This study investigated the effect of tube diameter on flow boiling characteristics of refrigerant R32 in horizontal small-diameter tubes with 1.0, 2.2, and 3.5 mm inner diameters. The boiling heat transfer coefficient and pressure drop were measured at 15 °C saturation temperature. The effects of mass velocity, heat flux, quality, and tube diameter were clarified. The flow pattern of R32 for adiabatic two-phase flow in a horizontal glass tube with an inner diameter of 3.5 mm at saturation temperature of 15 °C was investigated. Flow patterns such as plug, wavy, churn, and annular flows were observed. The heat transfer mechanisms of forced convection and nucleate boiling were similar to those in conventional-diameter tubes. In addition, evaporation heat transfer through a thin liquid film in the plug flow region for low quality, mass velocity, and heat flux was observed. The heat transfer coefficient increased with decreasing tube diameter under the same experimental condition. The fictional pressure drop increased with increasing mass velocity and quality and decreasing tube diameter. The experimental values of the heat transfer coefficient and frictional pressure drop were compared with the values calculated by the empirical correlations in the open literature.