Comprehensive correlations for heat transfer during condensation in conventional and mini/micro channels in all orientations

Two alternative correlations are presented for heat transfer during condensation in plain conventional channels as well as mini/micro channels in all orientations. These have been validated by comparison with a database that includes 33 fluids, diameters 0.10 to 49.0 mm, reduced pressures 0.0008 to 0.946, mass flux from 1.1 to 1400 kgm⁻²s⁻¹, various shapes (round, rectangular, triangular, etc.), all orientations (horizontal, vertical up and down, and angles in between). The data are from 136 data sets from 67 sources. While both correlations predict the 4063 data points with mean absolute deviation (MAD) of about 17%, their accuracy varies somewhat in different ranges. The same data are also compared to other correlations.     

Comprehensive correlation for dispersed flow film boiling heat transfer in mini/macro tubes

A general correlation for heat transfer during dispersed flow film boiling is presented which is applicable to horizontal and vertical tubes. It is based on the two-step model. It has been verified with data for 10 fluids in horizontal and vertical upflow. The fluids include refrigerants, hydrocarbons, cryogens, CO₂ and water. The range of data included diameters from 0.98 to 25.0 mm, reduced pressures from 0.0046 to 0.99, mass flux from 3.7 to 5176 kg m⁻² s⁻¹, and qualities from 0.1 to 2.96. The 1481 data points from 38 sources are predicted with mean absolute deviation of 19.4%. Several other correlations are also compared to the same data and found to have much larger 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.     

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%.     

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.     

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%.     

Heat transfer intensity at forced flow boiling of cryogenic liquids in tubes

A generalized correlation for heat transfer with forced flow boiling of cryogenic liquids (nitrogen, hydrogen and neon) in tubes is presented. The correlation describes, with a ± 35% accuracy, all known experimental data for vertical and horizontal channels (without stratified regimes) in the pressure range 0.9 – 22 bar, mass flow rates from 20 to 2200 kg m^?2 S^?1, and heat fluxes from 400 to 210 000 W m^?2. The boundary line between channels of a large and a small diameter has been established; a simple method of taking into account the higher heat transfer intensity in capillaries has been suggested.     

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.     

Transient burn-out in liquid helium with rapid rise of heat flux

This Paper presents the results of experimental and theoretical investigations of transient burn-out in liquid helium moving freely. Heat input in the open vertical channel with liquid helium was increased linearly. The rate of increase of heat flux was varied from 100 to 23 000 W m⁻² s⁻¹, ensuring both quasi-steady and transient conditions. When heat input rose rapidly, burn-out simultaneously appeared over the entire channel length. This can be explained by the peculiarities of unsteady hydrodynamics. A physical model of transient boiling burn-out in a pool and in channels is presented. Relationships for some forms of heat flux variation in time have been derived on the basis of this model. Present experimental data and results of other authors are verified by means of the developed model.     

A correlation for heat transfer during boiling on bundles of horizontal plain and enhanced tubes

A dimensionless correlation is presented for calculation of local heat transfer coefficients during saturated boiling in bundles of plain and enhanced tubes. It has been verified with a wide range of data that include 12 fluids (water, ammonia, halocarbon refrigerants, and hydrocarbons), plain and a variety of enhanced tubes of many materials, inline and staggered arrangements, pitch to diameter ratios 1.17 to 2.0, reduced pressures from 0.005 to 0.2866, mass flux from 0.17 to 1391 kgm⁻²s⁻¹, heat flux from 1 to 1000 kWm⁻², and vapor quality from 0 to 0.98. A total of 2173 data points from 51 data sets from 28 sources are predicted with mean absolute deviation of 15.2%.     

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.     

Experimental investigation on flow condensation of methane in a horizontal smooth tube

An experimental investigation on flow visualization of adiabatic and condensation conditions as well as condensation heat transfer coefficient and pressure drop of methane in a horizontal smooth tube was carried out. The tests were conducted at saturation pressure of 2–3.5 MPa with mass flux of 99–255 kg m⁻² s⁻¹ and fluid-to-wall temperature difference of 4.8–20.2 K throughout the vapor quality range. The effects of mass flux, saturation pressure, vapor quality and temperature difference were studied and discussed. In order to expand the range of temperature difference, some condensation heat transfer coefficients of ethane with larger temperature differences (19.7–39.2 K) were also reported in this paper. The experimental data were compared with many well-known correlations of condensation heat transfer coefficient and pressure drop. An improved heat transfer correlation for different flow patterns was proposed and predicted the experimental results well with a mean absolute relative deviation of 6.86%.     

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.     

A new model for flow boiling heat transfer coefficient inside horizontal tubes with twisted-tape inserts

This paper presents a new method to predict the heat transfer coefficient during flow boiling inside horizontal tubes containing twisted tape inserts. The method was developed based on the database presented by Kanizawa et al. This database comprises flow boiling results for horizontal tubes with internal diameters of 12.7 and 15.9 mm, twisted-tape ratios of 3, 4, 9 and 14, mass velocities ranging from 75 to 200 kg m⁻² s⁻¹, heat fluxes of 5 and 10 kW m⁻² and saturation temperatures of 5 and 15 °C. The method is flow-pattern based and considers flow boiling, dryout and mist flow regions. The predictive method also takes into account the physical picture of the swirl flow phenomenon by considering swirl flow effects promoted by the twisted tape insert. The proposed method provides satisfactory predictions and captures the main heat transfer trends of the data of Kanizawa et al. and also of independent data from literature.     

A research on the dryout characteristics of CO2'S flow boiling heat transfer process in mini-channels

The experimental and theoretical researches have been carried out to get the flow boiling heat transfer characteristics of carbon dioxide (CO₂ or R744) as a refrigerant in horizontal mini-channel. Based on infrared thermal imaging tests and experimental studies on heat transfer coefficients, the heat transfer coefficients and dryout characteristics of CO₂ are analyzed qualitatively and quantitatively in following conditions: Heat flux: 2~35 kW m^-2, Mass flux: 50~1350 kg m^-2 s^-1, saturation temperature: −10~15 °C, mini-channel inner diameter: 1 mm and 2 mm. Primary conclusions can be drawn from the results of the experiments: The increase of heat flux enhances the nucleate boiling heat transfer of the refrigerant inside mini-channel, which leads to the remarkable increase of heat transfer coefficient. But it speeds up the process of dryout. It also has a certain influence on vapor qualities of dryout at both the starting and the ending stage. The effect of mass flux on heat transfer enhancement depends on the dominant heat transfer mode in the tube. With the increase of mass flow rate, the vapor quality at the start of dryout has a decreasing trend. But the heat transfer coefficient increases at the end of dryout process or even after dryout process; the heat transfer coefficient does not vary monotonically with the saturation temperature: when the saturation temperature is high and even close to CO₂'s critical temperature, the heat transfer coefficient increases with the increase of saturation temperature; when the saturation temperature is low, the heat transfer coefficient increases with the decrease of saturation temperature. Besides, during the heat transfer process, the dryout vapor quality falls monotonically with the increase of saturation temperature. It is reasonable to conclude that dryout characteristics have significant influence on heat transfer coefficient. Fang correlation that predicts the heat transfer coefficient of CO₂ is in good agreement with the experimental data, which has a mean absolute deviation of 15.7%, and predicts 71.98% of the entire database within ±20% and 86.84% of the entire database within ±30%.     

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 downward flow boiling heat transfer characteristics of propane in shell side of LNG spiral wound heat exchanger

For designing LNG spiral wound heat exchangers (SWHE), the boiling heat transfer mechanism of two-phase hydrocarbon refrigerant flowing downward in shell side should be known. In this study, an explosion-proof experimental rig was established for measuring heat transfer coefficients (HTC) and observing flow patterns. The test section contains three-layer tube bundles to emulate the actual structure and flow conditions of an SWHE. Propane as one main component of shell-side refrigerant is used as the tested fluid. The experimental conditions cover heat fluxes of 4~10 kW⋅m⁻², mass fluxes of 40~80 kg (m²⋅s)⁻¹ and vapor qualities of 0.2~1.0. The results indicate that HTC initially increases and then decreases with the increment of vapor quality, representing a maximum at a vapor quality of 0.8~0.9; the effect of heat flux on HTC increases with the increment of heat flux. A correlation of HTC was developed covering 98% of the experimental data within a deviation of ±20%.     

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.     

Flow boiling of ammonia in a plain and a low finned horizontal tubeEbullition en éncoulement d'ammoniac dans un tube lisse ou un tube ailetté

Experimental data of the local heat transfer coeffcient of flow boiling ammonia in dependence of vapor fraction, mass flux and local heat flux is presented. Two horizontal test sections of 450 mm length and an inner diameter of 10 mm have been used, one being a plain tube, one being a spirally low finned tube. A constant wall temperature boundary has been aimed for the test section by heating with a fluid condensing on the tube outside. Local heat transfer coeffcients and pressure drops have been measured in the range −40 < T_sat < 4°C, 0 < x< 0.9, 50 < < 150 kg/m² s and 2 < ΔT_w < 15 K with resulting heat fluxes of 17 < < 75 kW/m². The vapor quality is denoted as x, is the mass flux and ΔT_w the wall superheat. The measured data is carefully evaluated using a finite element model of the tube with regard to the circumferential heat flow distribution. The smooth tube results are compared with recently published data and the correlation from Zürcher (Zürcher, O., Thome, J.R., Favrat, D. Evaporation of ammonia in a smooth horizontal tube: heat transfer measurements and predictions. Journal of Heat Transfer, 1999;121:89–101), and with the correlations of Steiner (Steiner D. Strömungssieden gesättigter Flüssigkeiten. VDI-Wärmeatlas, vol. 8. VDI-Verlag, 1997) and Kattan (Kattan N, Thome JR, Favrat D. Flow boiling in horizontal tubes: part 3 — development of a new heat transfer model based on flow pattern. Transactions of the ASME, 1998;120). The results of the low finned tube are not matched by any known correlation.