Claudio Zilio

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.

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.

An Assessment of Heat Transfer through Fins in a Fin-and-Tube Gas Cooler for Transcritical Carbon Dioxide Cycles

In CO2 transcritical refrigeration cycles, fin-and-tube coils are still considered possible gas cooling devices due to their lower cost when compared with recent aluminium minichannel heat exchangers. In spite of the very high working pressures, an off-the-shelf coil with four ranks of 3/8 in. (9.52 mm) copper tube and louvered fins was found suitable to work with high R-744 pressures and has been studied as a gas cooler in a test rig built for testing carbon dioxide (CO2) equipment operating with air as a secondary fluid. The test rig consists of two closed-loop air circuits acting as heat sink and heat source for the gas cooler and evaporator, respectively. The tested refrigerating circuit consists of two tube-and-fin heat exchangers as the gas cooler and the evaporator, a back-pressure valve as the throttling device, a double-stage compound compressor equipped with an oil separator, and an intercooler. A full set of thermocouples, pressure transducers, and flowmeters allows measurement and recording of all the main parameters of the CO2 cycle, enabling heat balance to be performed for both air side and refrigerant side. Tests focused on two different gas coolers, with continuous and cut fins, and on two different circuit arrangements. Tests on each heat exchanger were run at three different inlet conditions, for both CO2 and air. A simulation model was developed for this type of heat exchanger and three models (Dang and Hihara 2004; Gnielinski 1976; Pitla et al. 2002) proposed for the CO2 supercritical cooling heat transfer coefficients were implemented and compared in the code. The model results are compared with the experimental data for the finned coil; emphasis is given to the effect of heat conduction through fins between adjacent tube ranks on system efficiency. In the paper, the experimental results for transcritical CO2 entering the gas cooler at 87.0°C (7.911 MPa), 97.6°C (8.599 MPa), and 107.8°C (9.102 MPa) with air inlet temperatures of 20.3°C, 21.5°C, and 23.0°C, respectively, are presented. By using a coil with fins modified to reduce the heat conduction, a 3.7% to 5.6% heat flux improvement was gained. This improvement can be clearly translated in terms of coefficient of performance (COP), since a low value of the CO2 temperature at its outlet increases the cooling capacity. Considering a reference cycle with the same operating conditions, a 5.7% to 6.6% increase of COP can be obtained.

Optimisation of the throttling system in a CO2 refrigerating machine

This paper is an answer to the need of finding the optimal solution for the throttling system in refrigerating machines using CO2 as working fluid; such a solution must combine reliability, low installation cost and high energy efficiency. To this purpose, different expansion systems are compared by means of a simulation programme, including a new one, operating with a differential valve, a liquid receiver and a thermostatic valve. The typical compression refrigerating cycle performed by CO2 involves transcritical operations and therefore the upper pressure needs to be adjusted to the optimal value, that, unlike the traditional cycle, is not determined by heat transfer. The innovative system here proposed shows an intrinsic self-adjusting capability that leads to COP values quite close to the maximum ones when a fixed suitable value of the differential pressure is chosen, even if the temperature of the secondary fluid varies to a large extent.

Air Flow in Aluminum Foam: Heat Transfer and Pressure Drops Measurements

Abstract Metal foams are cellular structure materials that present open cells, randomly oriented and mostly homogeneous in size and shape. Cellular structure materials, and particularly open-cell metal foams, have been proposed as possible substitutes for traditional finned surfaces in electronics cooling applications. This article presents the heat transfer and pressure drops measurements obtained during air flow through an aluminum foam, which has 40 pores per inch with 0.63 mm pore diameter. The specimen has been inserted in a new open-circuit type wind channel with a rectangular cross-section that has recently been built at the Department of Fisica Tecnica of the University of Padova. The experimental heat transfer coefficients and pressure drops have been collected by varying the air flow rate supplied by the screw compressor that provides a variable volumetric air flow ranging between 0–90 m3h−1 at a constant gauge pressure of 7 bar. The specific heat flux has been simulated by powering with a 25-kWm−2 copper heater attached at the bottom of the aluminum foam base plate. The experimental results are reported in terms of heat transfer coefficients, mean normalized wall temperatures, and pressure drops.

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

Two-phase pressure drop and condensation heat transfer of R32/R1234ze(E) non-azeotropic mixtures inside a single microchannel

Much attention has been paid in the recent years to the possible use of fluorinated propene isomers for the substitution of high global warming potential refrigerants. Among the fluorinated propene isomers, R1234ze(E) may be a substitute of R134a for refrigeration applications. R1234ze(E) has a global warming potential lower than 1 (considering a period time of 100 years), and it is receiving some attention also as a component of low global warming potential mixtures. In this article, a mixture of R1234ze(E) and R32 has been investigated at two different mass compositions (23/77% and 46/54% by mass) with regard to the performance at the condenser. The local heat transfer coefficients during condensation in a single microchannel with 0.96 mm diameter are measured and analyzed. The frictional two-phase pressure drop in the same channel is also investigated. The present tests are carried out on the experimental apparatus available at the Two-Phase Heat Transfer Lab of the University of Padova. The new experimental data are compared to those of pure R1234ze(E) and R32. This allows the heat transfer penalization due to the mass transfer resistance occurring during condensation of these zeotropic mixtures to be analyzed and suitable predicting models to be assessed. The knowledge of heat transfer coefficient and pressure drop allows evaluation of the overall performance of these mixtures when used in condensers.

Thermophysical properties and heat transfer and pressure drop performance potentials of hydrofluoro-olefins, hydrochlorofluoro-olefins, and their blends

This article considers the heat transfer and pressure drop performance potentials of halogenated propene isomers during in-tube condensation and in-tube flow boiling using the penalty factor and total temperature penalization concepts, respectively. In particular, five isomers are considered: R-1233xf, R-1233zd(E), R-1234yf, R-1234ze(E), and R-1243zf. In addition, to these five pure fluids, the heat transfer and pressure drop performance potentials are investigated for five R-32/R-1234yf blends and for twenty-seven blends being considered as part of the Air-Conditioning, Heating, and Refrigeration Institute (AHRI) Low-GWP Alternative Refrigerants Evaluation Program. The article also presents thermophysical property estimations for the five pure fluids relative to R-134a or R-123, and the five R-32/R-1234yf blends relative to R-134a. The thermophysical properties considered are the ones that influence the heat transfer and pressure drop performance potentials, and include the thermodynamic properties temperature, pressure, density, latent heat of vaporization, and specific heat, and the transport properties thermal conductivity and viscosity. The article also presents a literature review of relevant articles for condensation and boiling heat transfer and pressure drop of fluorinated propene isomers.