Luca Doretti

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.

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.

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.

A New Model for Forced-Convection Condensation on Integral-Fin Tubes

Integral-fin tubes are extensively used in shell-and-tube condensers for refrigeration. This work investigates the effects of vapor shear during pure vapor external condensation on horizontal integral-fin tubes. More than 220 experimental data-points in a wide range of operative conditions and enhanced surface geometries are reported together with the visual observation of the condensate flow patterns. The effects of vapor shear are relevant only for vapor Reynolds numbers greater than 70,000--100,000, while heat transfer enhancement is linked to the geometry of the extended surface. A simple semi-empirical equation was developed to account for the shear stress contribution in forced-convection condensation: this equation, applied in conjunction with the model by Briggs and Rose (1994) for stationary vapor condensation, displays a good ability in reproducing all the available data with relevant vapor velocities.

A tube-in-tube water/zeotropic mixture condenser: design procedure against experimental data

Abstract The object of the present paper is related to the design of condensers for the non-azeotropic mixtures of refrigerants. The high temperature glide mixture R-125/236ea at three different compositions (0.30/0.70, 0.46/0.54, 0.64/0.36 by mass) was tested during condensation inside a 2 m long smooth horizontal tube-in-tube exchanger. The superheated vapour entering the tube is first cooled and then condensed against cold water flowing in the annulus. The experimental data, taken at 400 and 750 kg /( m 2 s ) mass velocity, is used for comparison against the method of Colburn and Drew [Trans. AIChemE, 33 (1937) 197]. It is shown that this method gives a reasonably good prediction of the heat flux exchanged in the tube.

Condensation heat transfer inside multi-port minichannels

In this paper the experimental heat transfer coefficients measured during condensation of R134a and R410A inside multiport minichannels are presented. 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. Heat transfer data are compared against models to show the accuracy of the models in the prediction of heat transfer coefficients inside minichannels.Copyright

Experimental Investigation on Condensation Heat Transfer Coefficient Inside Multi-Port Minichannels

Very little experimental information is available in the open literature about condensation inside minichannels. Most of the experimental work has been carried out by using the Wilson plot technique. This method is simple to implement because it does not require the direct measurement of the tube wall temperature. However it becomes inaccurate when a small thermal resistance is present on the test side as compared to the opposite (cooling) side, which is actually the case with a multichannel tube at high values of the internal heat transfer coefficient. In fact, in a multi-port tube internal webs work as fins, and their efficiency is close to unity; thus the internal heat transfer area is higher than the external one. In this paper a new technique to measure the heat transfer coefficient during condensation inside a multi-port extruded minichannel tube is presented. Some R134a preliminary data is also reported.Copyright