Davide Del Col

Condensation in Horizontal Smooth Tubes: A New Heat Transfer Model for Heat Exchanger Design

This paper proposes a new method to determine the condensation heat transfer coefficient of fluids flowing into horizontal smooth tubes with internal diameters D > 3 mm. The method has been drawn up as simply as possible and is ready to use in heat exchanger modeling and design applications. It is also suitable to work very well with old and new fluids used in the refrigeration, air conditioning, and heat pump industries. Particular attention is given to accuracy: it has been tested over a wide updated experimental database and comes from many different independent researchers with reduced experimental uncertainties. In order to obtain an easy structure, only two equations are employed, related respectively to & Delta; T-independent and to & Delta; T-dependent fluid flows. All the parameters that influence the condensation heat transfer have been included. A comparison has been conducted against HCFCs, HFCs, HCs, carbon dioxide, ammonia, and water data. Zeotropic mixtures with two and three components are also considered in the comparison by applying the Bell and Ghaly [1] correction to calculate the relative heat transfer penalization. A model has been developed with the idea of getting high accuracy through an easy structure, and the results show a very satisfactory agreement with experimental data: average deviation eR = +2%, absolute mean deviation eAB = 14%, and standard deviation σN = 19% for the total number of 5478 data points.

Condensation inside and outside smooth and enhanced tubes — a review of recent research

Condensation heat transfer, both inside and outside horizontal tubes, plays a key role in refrigeration, air conditioning and heat pump applications. In the recent years the science of condensation heat transfer has been severely challenged by the adoption of substitute working fluids and new enhanced surfaces for heat exchangers. Well-known and widely established semiempirical correlations to predict heat transfer during condensation may show to be quite inaccurate in some new applications, and consequently a renewed effort is now being dedicated to the characterisation of flow conditions and associated predictive procedures for heat transfer and pressure drop of condensing vapours, even in the form of zeotropic mixtures. This paper critically reviews the most recent results appeared in the open literature and pertinent to thermal design of condensers for the air conditioning and refrigeration industry; both in-tube and bundle condensation are considered, related to the use of plain and enhanced surfaces.

Experimental investigation on condensation heat transfer and pressure drop of new HFC refrigerants (R134A, R125, R32, R410A, R236ea) in a horizontal smooth tube

Abstract This paper reports experimental heat transfer coefficients and pressure drops measured during condensation inside a smooth tube when operating with pure HFC refrigerants (R134a, R125, R236ea, R32) and the nearly azeotropic HFC refrigerant blend R410A. Data taken when condensing HCFC-22 are also reported for reference. The experimental runs are carried out at a saturation temperature ranging between 30 and 50°C, and mass velocities varying from 100 to 750 kg/(m2 s), over the vapour quality range 0.15–0.85. The effects of vapour quality, mass velocity, saturation temperature and temperature difference between saturation and tube wall on the heat transfer coefficient are investigated by analysing the experimental data. A predictive study of the condensation flow patterns occurring during the tests is also presented. Finally comparisons with predictions from the model by Kosky and Staub (Kosky PG, Staub FW. Local condensing heat transfer coefficients in the annular flow regime. AIChE J 1971;17:1037) are reported for all the data sets.

Heat transfer and pressure drop during condensation of refrigerants inside horizontal enhanced tubes

Abstract This paper presents a critical review of correlations to compute heat transfer coefficients and pressure drop, for refrigerants condensing inside commercially available tubes with enhanced surfaces of various types, and a theoretical analysis of the condensation phenomenon. Predictions from some of the above equations are compared with experimental data. In addition, information is presented about the influence of small amounts of compressor oil on the condensation of refrigerants in enhanced tubes.

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.

Numerical Simulation of Laminar Liquid Film Condensation in a Horizontal Circular Minichannel

A three-dimensional volume of fluid (VOF) simulation of condensation of R134a inside a 1 mm i.d. minichannel is presented. The minichannel is horizontally oriented and the effect of gravity is taken into account. Simulations have been run both with and without taking into account surface tension. A uniform interface temperature and a uniform wall temperature have been fixed as boundary conditions. The mass flux is G = 100 kg m−2 s−1 and it has been assumed that the flow is laminar inside the liquid phase while turbulence inside the vapor phase has been handled by a modified low Reynolds form of the k–ω model. The fluid is condensed till reaching 0.45 vapor quality. The flow is expected to be annular without the presence of waves, therefore the problem was treated as steady state. Computational results displaying the evolution of vapor–liquid interface and heat transfer coefficient are reported and validated against experimental data. The condensation process is found to be gravity dominated, while the global effect of surface tension is found to be negligible. At inlet, the liquid film is thin and evenly distributed all around the tube circumference. Moving downstream the channel, the film thickness remains almost constant in the upper half of the minichannel, while the film at the bottom of the pipe becomes thicker because the liquid condensed at the top is drained by gravity to the bottom.

Condensation in a Square Minichannel: Application of the VOF Method

A number of steady-state numerical simulations of condensation of R134a at mass fluxes of 400 kg m−2 s−1 and 800 kg m−2 s−1 inside a 1-mm square cross section minichannel are proposed here and compared against simulations in a circular cross section channel with the same hydraulic diameter. The volume of fluid (VOF) method is used to track the vapor–liquid interface, and the effects of interfacial shear stress, surface tension, and gravity are taken into account. A uniform wall temperature is fixed as a boundary condition. At both mass velocities the liquid film and the vapor core are treated as turbulent; a low-Re form of the SST k-ω model has been used for the modeling of turbulence through both the liquid and vapor phases. Numerical simulations are validated against experimental data. The influence of the surface tension on the shape of the vapor–liquid interface may provide some heat transfer enhancement in a square cross section minichannel, but this depends on the mass flux and it may be not significant at high mass velocity, as confirmed by experimental data and by the present numerical work. The gravity force is shown to be responsible for the liquid film thickness increase at the bottom of the channel in the circular cross section, but the gravity force has a minor effect in the square minichannel; at these mass velocities, the heat transfer mechanism is dominated by shear stress and surface tension.

Condensation of Zeotropic Mixtures in Horizontal Tubes: New Simplified Heat Transfer Model Based on Flow Regimes

Reference LTCM-ARTICLE-2005-001View record in Web of Science Record created on 2005-07-06, modified on 2017-05-10

Condensation Heat Transfer and Pressure Losses of High- and Low-Pressure Refrigerants Flowing in a Single Circular Minichannel

A 0.96 mm circular minichannel is used to measure both heat transfer coefficients during condensation and two-phase pressure losses of the refrigerants R32 and R245fa. Test runs have been performed at around 40°C saturation temperature, corresponding to 24.8 bar saturation pressure for R32 and 2.5 bar saturation pressure for R245fa. The pressure drop tests have been performed in adiabatic flow conditions, to measure only the pressure losses due to friction. The heat transfer experimental data are compared against predicting models to provide a guideline for the design of minichannel condensers.

Pressure Drop During Two-Phase Flow of R134a and R32 in a Single Minichannel

Condensation in minichannels is widely used in air-cooled condensers for the automotive and air-conditioning industry, heat pipes, and compact heat exchangers. The knowledge of pressure drops in such small channels is important in order to optimize heat transfer surfaces. Most of the available experimental work refers to measurements obtained within multiport smooth extruded tubes and deal with the average values over the number of parallel channels. In this context, the present authors have set up a new test apparatus for heat transfer and fluid flow studies in single minichannels. This paper presents new experimental frictional pressure gradient data, relative to single-phase flow and adiabatic two-phase flow of R134a and R32 inside a single horizontal minitube, with a 0.96 mm inner diameter and with not-negligible surface roughness. The new all-liquid and all-vapor data are successfully compared against predictions of single-phase flow models. Also the two-phase flow data are compared against a model previously developed by the present authors for adiabatic flow or flow during condensation of halogenated refrigerants inside smooth minichannels. Surface roughness effects on the liquid-vapor flow are discussed. In this respect, the friction factor in the proposed model is modified, in order to take into consideration also effects due to wall roughness.