Condensation Heat Transfer and Pressure Gradient Inside Multiport Minichannels

Abstract

In this paper, the experimental heat transfer coefficients measured during condensation of R134a and R410A inside multiport minichannels are presented. The frictional pressure gradient was also measured during adiabatic two-phase flow. The need for experimental research on condensation inside multiport minichannels comes from the wide use of those channels in automotive air-conditioners. The perspective for the adoption of similar channels in the residential air conditioning applications also calls for experimental research on new high pressure refrigerants, such as R410A.

Experimental data are compared against models to show the accuracy of the models in the prediction of heat transfer coefficients and pressure drop inside minichannels.     

Two-phase flow in high-heat-flux micro-channel heat sink for refrigeration cooling applications: Part I--pressure drop characteristics

Two-phase pressure drop was measured across a micro-channel heat sink that served as an evaporator in a refrigeration cycle. The micro-channels were formed by machining 231 μm wide × 713 μm deep grooves into the surface of a copper block. Experiments were performed with refrigerant R134a that spanned the following conditions: inlet pressure of P_in = 1.44-6.60 bar, mass velocity of G = 127-654 kg/m² s, inlet quality of x_e,in = 0.001-0.25, outlet quality of x_e,out = 0.49-superheat, and heat flux of q″ = 31.6-93.8 W/cm². Predictions of the homogeneous equilibrium flow model and prior separated flow models and correlations yielded relatively poor predictions of pressure drop. A new correlation scheme is suggested that incorporates the effect of liquid viscosity and surface tension in the separated flow model’s two-phase pressure drop multiplier. This scheme shows excellent agreement with the R134a data as well as previous micro-channel water data. An important practical finding from this study is that the throttling valve in a refrigeration cycle offers significant stiffening to the system, suppressing the large pressure oscillations common to micro-channel heat sinks.     

Comparative investigations of the condensation of R134a and R404A refrigerants in pipe minichannels

The present paper describes the results of experimental investigations of heat transfer and pressure drop during the condensation of the R134a and R404A refrigerants in pipe minichannels with internal diameters d = 0.31–3.30 mm. The results concern investigations of the local heat transfer coefficient and a pressure drop in single mini-channels. The results were compared with calculations according to the correlations proposed by other authors. Within the range of the examined parameters of the condensation process in mini-channels produced from stainless steel, it was established that the values of the heat transfer coefficient may be described with Akers et al. and Shah correlations within a limited range of the mass flux density of the refrigerant and the mini-channel diameter. A pressure drop during the condensation of these refrigerants is described in a satisfactory manner with Friedel and Garimella correlations. On the basis of the experimental investigations, the authors proposed their own correlation for the calculation of local heat transfer coefficient α_x.     

Update on Condensation Heat Transfer and Pressure Drop inside Minichannels

The present paper reviews published experimental work focusing on condensation flow regimes, heat transfer, and pressure drop in minichannels. New experimental data are available with high (R410A), medium (R134a), and low (R236ea) pressure refrigerants in minichannels of different cross-section geometries and with hydraulic diameters ranging from 0.4 to 3 mm. Because of the influence of flow regimes on heat transfer and pressure drop, a literature review is presented to discuss flow regimes transitions. The available experimental frictional pressure gradients and heat transfer coefficients are compared with semi-empirical and theoretical models developed for conventional channels and models specifically created for minichannels. Starting from the results of the comparison between experimental data and models, the paper will discuss and evaluate the opportunity for a new heat transfer model for condensation in minichannels; the new model attempts to take into account the effect of the entrainment rate of droplets from the liquid film.     

Numerical simulation of condensation for R410A at varying saturation temperatures in mini/micro tubes

ABSTRACT

Heat transfer and pressure drop characteristics of condensation for R410A inside horizontal tubes (d_h = 0.25, 1, and 2 mm) at saturation temperatures Tsat = 310, 320, and 330 K are investigated numerically. The results indicate that local heat transfer coefficients and pressure drop gradients increase with increasing mass flux and vapor quality and with decreasing tube diameter and saturation temperature. Liquid film thickness also increases with increasing saturation temperature because of the lower surface tension at higher saturation temperature. When gravity dominates the condensation process, a vortex with its core lying at the bottom of the tube is found in the vapor phase region. For the annular flow regime, stream traces point from the symmetry plan to the liquid–vapor interface, where the vapor phase becomes the liquid phase. Numerical heat transfer coefficients and pressure drop gradients are compared to available empirical correlations. Two new models for heat transfer coefficients and frictional pressure drop gradients are developed based on the numerical work.     

Experimental study on condensation heat transfer enhancement and pressure drop penalty factors in four microfin tubes

Heat transfer and pressure drop characteristics of four microfin tubes were experimentally investigated for condensation of refrigerants R134a, R22, and R410A in four different test sections. The microfin tubes examined during this study consisted of 8.92, 6.46, 5.1, and 4 mm maximum inside diameter. The effect of mass flux, vapor quality, and refrigerants on condensation was investigated in terms of the heat transfer enhancement factor and the pressure drop penalty factor. The pressure drop penalty factor and the heat transfer enhancement factor showed a similar tendency for each tube at given vapor quality and mass flux. Based on the experimental data and the heat-momentum analogy, correlations for the condensation heat transfer coefficients in an annular flow regime and the frictional pressure drops are proposed.     

Correlation of the Flow Pattern and Flow Boiling Heat Transfer in Microchannels

Flow boiling in microchannels is characterized by the considerable influence of capillary forces and constraint effects on the flow pattern and heat transfer. In this article we utilize the features of gas–liquid flow patterns in rectangular microchannels under adiabatic conditions to explain the regularities of refrigerants flow boiling heat transfer. The flow-pattern maps for the upward and horizontal nitrogen–water flow in a microchannel with the size of 1500 × 720 μm were determined via dual-laser flow scanning and compared with corrected Mishima and Ishii prediction. Flow boiling heat transfer was studied for vertical and horizontal microchannel heat sink with similar channels using refrigerants R-21 and R-134a. The data on local heat transfer coefficients were obtained in the range of mass flux from 33 to 190 kg/m²-s, pressure from 1.5 to 11 bar, and heat flux from 10 to 160 kW/m². The nucleate and convective flow boiling modes were observed for both refrigerants. It was found that heat transfer deterioration occurred for annular flow when the film thickness became small to suppress nucleate boiling. The mechanism of heat transfer deterioration was discussed and a model of heat transfer deterioration was applied to predict the experimental data.     

Condensation Flow Mechanisms in Microchannels: Basis for Pressure Drop and Heat Transfer Models

This paper presents an overview of the use of flow visualization in micro- and mini-channel geometries for the development of pressure drop and heat transfer models during condensation of refrigerants. Condensation flow mechanisms for round, square, and rectangular tubes with hydraulic diameters in the range of 1–5 mm for 0 < x < 1, and 150 kg/m²-s and 750 kg/m²-s were recorded using unique experimental techniques that permit flow visualization during the condensation process. The effect of channel shape and miniaturization on the flow regime transitions was documented. The flow mechanisms were categorized into four different flow regimes: intermittent flow, wavy flow, annular flow, and dispersed flow. These flow regimes were further subdivided into several flow patterns within each regime. It was observed that the intermittent and annular flow regimes become larger as the tube hydraulic diameter is decreased, and at the expense of the wavy flow regime. These maps and transition lines can be used to predict the flow regime or pattern that will be established for a given mass flux, quality, and tube geometry. These observed flow mechanisms, together with pressure drop measurements, are being used to develop experimentally validated models for pressure drop during condensation in each of these flow regimes for a variety of circular and noncircular channels with 0.4 < D_h < 5 mm. These flow regime-based models yield substantially better pressure drop predictions than the traditionally used correlations that are primarily based on air-water flows for large diameter tubes. Condensation heat transfer coefficients were also measured using a unique thermal amplification technique that simultaneously allows for the accurate measurement of the low heat transfer rates over small increments of refrigerant quality and high heat transfer coefficients characteristic of microchannels. Models for these measured heat transfer coefficients are being developed using the documented flow mechanisms and the corresponding pressure drop models as the basis.     

An experimental investigation on pressure drop of steam condensing in silicon microchannels

Experiments are carried out to study the two-phase pressure drop for water vapor condensation in four smooth trapezoidal silicon microchannels having hydraulic diameters of 109 μm, 142 μm, 151 μm, and 259 μm, respectively. It is found that two-phase frictional pressure drops in the microchannels are greatly influenced by the hydraulic diameter, mass flux and vapor quality. The two-phase pressure drop data in microchannels are compared with existing correlations for macro- and mini-channels based on the homogenous model and the separated flow model to determine their applicability to condensing flows in microchannels. A modified correlation for the Matinelli–Chisholm constant, taking into consideration of surface tension and diameter effects, is developed in the form of the Lockhart–Martinelli correlation for the pressure drop in steam condensation in microchannels. The resulting condensation pressure drop correlation equation is within ±15% of the experimental data.     

An Experimental Study of Flow Condensation Heat Transfer Inside Circular and Rectangular Mini-Channels

Abstract

By using unique experimental techniques and the careful construction of an experimental apparatus, the characteristics of the local heat transfer were investigated using the condensing R134a two-phase flow in horizontal single mini-channels. The circular channels (D _h = 0.493, 0.691, and 1.067 mm) and rectangular channels (Aspect Ratio = 1.0; D _h = 0.494, 0.658, and 0.972 mm) were tested and compared. Tests were performed for a mass flux of 100, 200, 400, and 600 kg/m²s, a heat flux of 5 to 20 kW/m², and a saturation temperature of 40°C. In this study, the effect of heat flux, mass flux, vapor qualities, hydraulic diameter, and channel geometry on flow condensation were investigated, and the experimental local condensation heat transfer coefficients are shown. The experimental data of condensation Nusselt number are compared with existing correlations.     

Fluidization characteristics of two- and three-dimensional fluidized beds

Experiments have been conducted with particles of sand (d_p = 1,421 μm, silica sand (d_p = 171 μm), glass beads (d_p = 2,745 μm) and wax pellets (d_p = 5,476 μm) in two- and three-dimensional fluidized beds. In both beds, measurement of pressure drop as a function of superficial air velocity are taken and employed to determine the minimum fluidization velocity and the variation of bed voidage with superficial air velocity. Data analysis has made it possible to understand the nature of frictional forces that the front and back bounding walls of two-dimensional fluidized beds caused on bed particles, as well as their dependence on particle shape. Information useful in the design of fluidized beds has been generated.     

Heat transfer and pressure drop during flow boiling of pure refrigerants and refrigerant/oil mixtures in tube with porous coating

The experimental stand and procedure for flow boiling investigations are described. Experimental data for pure R22, R134a, R407C and their mixtures with polyester oil FUCHS Reniso/Triton SEZ 32 in a tube with porous coating and smooth, stainless steel reference tube are presented. Mass fraction of oil was equal to 1% or 5%. During the tests inlet vapour quality was set at 0 and outlet quality at 0.7. Mass velocity varied from about 250 to 500 kg/m²s. The experiments have been conducted for average saturation temperature 0 °C. In the case of flow boiling of pure refrigerants, the application of a porous coating on inner surface of a tube results in higher average heat transfer coefficient and simultaneously in lower pressure drop in comparison with the flow boiling in a smooth tube for the same mass velocity. Correlation equation for heat transfer coefficient calculation during the flow boiling of pure refrigerants inside a tube with porous coating has been proposed.     

Effect of Blocked Core-Tube Diameter on Heat Transfer Performance of Internally Longitudinal Finned Tubes

Three-dimensional turbulent flow and heat transfer in an internally finned tube with a blocked core-tube have been numerically studied by the realizable k ? ε turbulence model with the wall-function method. The numerical method is validated by comparing the calculated results with experimental data. The range of ratio of blocked core-tube outside diameter to outer-tube inside diameter (d ₀/D _i ) is from 0.25 to 0.75. The computational results demonstrated that there exists an optimal ratio of (d ₀/D _i ) under both identical mass flow rate and identical pressure drop. The optimal ratio of (d ₀/D _i ), which is reduced with the increase of mass flow rate, is approximately 0.5 to 0.625 at given mass flow rate for both constant wall temperature and uniform wall heat flux. The optimal ratio of (d ₀/D _i ) at a given pressure drop is from 0.44 to 0.50, which is also slightly reduced with the increase of pressure drop. Furthermore, the optimal ratio of (d ₀/D _i ) is not sensitive to the number of cross-section wavy fins of an internally longitudinal finned tube, in the range of a fin wave number of 15–25.     

Thermodynamic property formulations and heat transfer aspects for replacement refrigerants: R-123 and R-134a

This paper reports analytical relations for the thermodynamic properties enthalpy, entropy, heat capacities at constant pressure and temperature of the replacement refrigerants R-123 and R-134a. These refrigerants are considered promising as substitutes for the fluids R-11 and R-12, respectively, which are two of the most widely used CFC refrigerants. In addition to the properties, the three real gas isentropic exponents k_p,v,k_v,T, k_p,T are calculated, which may be used instead of the classical exponent k=c_p/c_v in the ideal gas isentropic change equations to describe with good accuracy the real gas behaviour. A systematic study to research the influence of various parameters on heat transfer during condensation of R-123 and R-134a on horizontal integral-fin tubes is also carried out. The results are useful in refrigeration applications to improve the basic design, as a significant concern about new refrigerants to replace the CFCs has increased very rapidly due to the destruction of stratospheric ozone and global warming. © 1997 John Wiley & Sons, Ltd.     

High-Pressure Condensing Refrigerant Flows through Microchannels,Part I: Pressure Drop Models

ABSTRACT

Condensation of high-pressure refrigerants in small-diameter channels over a wide range of reduced pressures approaching the critical point is investigated in this two-part study. In this paper, Part I of the study, a multi-regime pressure drop model for condensing fluids in small-diameter channels is presented. Pressure drop measurements were conducted on condensing R404A in circular channels (inner diameter = 0.508, 1.00, 3.05 mm) over the entire quality range. Saturation temperatures were varied from 30 to 60°C, and mass fluxes from 200 to 800 kg m^?2 s^?1, to evaluate their effect on pressure drop. The saturation temperatures investigated here correspond to reduced pressures between 0.38 and 0.77. The pressure drop models are developed using a microchannel flow regime map and the void fraction models presented by the authors in previous work. The resulting model predicts 85.5% of the data within ±25%. Part II of the study presents a corresponding heat transfer model.     

Prediction of pressure drop during horizontal annular flow boiling of pure and mixed refrigerants

An experimental study on pressure drop during horizontal flow boiling of pure and mixed refrigerants of R22, R114, R12, and R152a is reported. More than 600 pressure drop data are taken for annular flow under uniform heat flux at reduced pressures of 0.08–0.16 (200–800 kPa). The range of heat flux and mass flow rate is 10–45 kW m⁻² and 16–46 g s⁻¹ (corresponding to 230–720 kg m⁻² s⁻¹ in terms of mass flux). The results are compared against well-known correlations; Bo Pierre's correlation failed to correlate half of the present data while Martinelli and Nelson's correlation overpredicted it by 20%. Pressure drops with both pure and mixed refrigerants, however, are well correlated by Martinelli's parameter, X_tt. Furthermore, no composition dependence of pressure drop is found with mixtures. A simple correlation adopting the thermodynamic corresponding states principles is developed with a chart to facilitate the estimation of pressure drop during flow boiling. The prediction agrees with the measured data with a mean deviation of 8.4%.     

Prediction of capillary tubes with alternative refrigerants to CFC‐502

A numerical model is presented in this paper, for predicting capillary tube performance using new alternative refrigerants to CFC‐502. The model has been established after the fluid flow conservation equations written for a homogeneous azeotropic refrigerant fluid flow under saturated, sub‐cooled and two‐phase conditions. The study was limited to the following azeotropic mixtures; R‐507, R‐404A, and quaternary mixture (R32/R125/R134a/R143a). Numerical results showed that the proposed model in question fairly simulated our experimental data and fairly predicted the capillary tube behaviour under different conditions. The results also indicated that a system using R‐507 would experience smaller pressure drop across the capillary compared to the other alternatives under question. Copyright © 2001 John Wiley & Sons, Ltd.     

Interactions Between Parallel Unevenly Heated Minichannels During Flow Boiling of R134A

Transient pressure drop of individual channels during flow boiling of R134a in four 0.54 mm square parallel minichannels was experimentally studied in this work. The design of the test section enabled the experimenter to control and to vary heat flux independently in each channel in the range from 3.82 to 18.66 kW/m² at five different overall flow rates from 86 to 430 kg/m²-s. Flow rate fluctuation in parallel channels due to the formation of bubbles under the nonuniform heat flux conditions caused significant oscillations in local pressure drop. Statistical analysis indicated that the pressure drop signal was normally distributed when boiling was stable with no incoming flow disturbance. Pressure drop distribution was highly skewed and multimodal when significant evaporation rate at low mass fluxes led to rapid annular flow formation, reducing the free flow of incoming fluid. Cross-correlation analysis revealed a strong interaction between minichannels having the highest heat flux difference among the set of channels. The least heated channel was more sensitive to the fluctuations in other channels. Cross-correlation between the most heated channel and the adiabatic one was estimated to be 39% when the total flow rate was the lowest, 86 kg/m²-s. The power of the relationship between channels dropped significantly as the flow rate increased. Less than 5% of data points could be considered cross-correlated at the highest flow rate of 430 kg/m²-s. Increasing the two-phase pressure drop across each channel caused higher resistance to the incoming disturbances and led to less interchannel interaction. This study of the channels interaction in a system of parallel, nonuniformly heated minichannels can be used as a tool to identify and quantify instabilities and reversed flow conditions.     

Flow boiling and condensation of tetrary refrigerant mixtures inside water/refrigerant enhanced surface tubing

This work presents the results of an experimental study concerning the heat transfer characteristics of two-phase flow condensation and boiling of tetrary (R-32/R-125/R143a/R134a) refrigerant mixtures inside water/refrigerant horizontal enhanced surface tubing. Heat transfer characteristics such as average heat transfer coefficients, as well as pressure drops of the tetrary refrigerant mixtures, have been predicted and compared with other mixtures during flow condensation and boiling inside enhanced surface tubing. It was found that the tetrary refrigerant blend has higher transfer coefficients than R-502, and the lowest pressure drop among the refrigerants studied. © 1997 by John Wiley & Sons, Ltd.     

A model application to study circuiting and operation in CO2 refrigeration coils

In this paper a previously developed model has been used to show that there are cases where refrigerant circuiting is necessary. Most hydro fluorocarbons can accommodate only limited tube lengths due to excessive pressure drop in refrigeration coils as is shown here with R507A, commonly employed in supermarkets. A study of the effect of circuiting on coil operation has then been performed for three different configurations of evaporation paths, using CO₂ as a working fluid. The basic unit consisted of a single circuit forming the whole coil unit. The other two configurations had, respectively, two and three circuits corresponding to the same total area and tube length as the basic unit. Comparison between these three units was made using the basic unit as a reference. The analysis was based on the numerical model that calculates incrementally parameter distributions for air across the coil and for refrigerant inside the tubes. Circuiting which consisted in defining refrigerant flow paths and varying flow rates within the coil was shown to affect performance and general operational coil behaviour, particularly the refrigerant pressure drop and the corresponding temperature glides. It is possible to use longer circuits with CO₂ (in comparison to other refrigerants), therefore reducing their number for a given capacity.