Assessment of boiling heat transfer correlations in the modelling of fin and tube heat exchangers

A new way to assess the performance of refrigeration system models is presented in this paper, based on the estimation of cycle parameters, such as the evaporation temperature which will determine the validity of the method. This paper is the first of a series which will also study the influence of the heat transfer coefficient models on the estimation of the refrigeration cycle parameters. It focuses on fin and tube evaporators and includes the dehumidification process of humid air. The flow through the heat exchanger is considered to be steady and the refrigerant flow inside the tubes is considered one-dimensional. The evaporator model is discretised in cells where 1D mass, momentum and energy conservation equations are solved by using an iterative procedure called SEWTLE. This procedure is based on decoupling the calculation of the fluid flows from each other assuming that the tube temperature field is known at each fluid iteration. Special attention is paid to the correlations utilised for the evaluation of heat transfer coefficients as well as the friction factor on the air and on the refrigerant side. A comparison between calculated values and measured results is made on the basis of the evaporation temperature. The experimental results used in this work correspond to an air-to-water heat pump and have been obtained by using R-22 and R-290 as refrigerants.     

A phenomenological model for analyzing reciprocating compressors

A new model for hermetic reciprocating compressors is presented. This model is able to predict compressor efficiency and volumetric efficiency in terms of a certain number of parameters (10) representing the main sources of losses inside the compressor. The model provides users with helpful information about the way in which the compressor is designed and working.A statistical fitting procedure based on the Monte Carlo method was developed for its adjustment. The model can predict compressor performance at most points with a maximum deviation of 3%.A possible gas condensation on cold spots inside the cylinder during the last part of the compression stroke was also evaluated.     

Liquid side heat transfer and pressure drop in finned-tube cooling-coils operated with secondary refrigerants

In this study full-scale experiments with two different conventional cooling-coils aimed for display cabinets were performed. Heat transfer and pressure drop on the liquid side for three different single phase secondary refrigerants were studied and compared to predictions by existing correlations. Predominantly, the laminar flow regime was studied. The results show that when predicting the heat transfer performance on the liquid side of a cooling-coil the Gnielinski correlation for thermally developing flow and uniform wall temperature boundary conditions (T) leads to good agreement for 0.0014 < x^* < 0.017 if 50 < Re < 1700, assuming a new entrance length is formed after each U-bend. In addition, these entrance lengths must also be accounted for, when predicting the pressure drop on the liquid side of the cooling-coil. The uncertainty of measurement can be a problem in this type of investigations and this has been taken into consideration when analysing the results.     

Assessment of boiling and condensation heat transfer correlations in the modelling of plate heat exchangers

This paper studies refrigeration cycles in which plate heat exchangers are used as either evaporators or condensers. The performance of the cycle is studied by means of a method introduced in previous papers which consists of assessing the goodness of a calculation method by looking at representative variables such as the evaporation or the condensation temperature depending on the case evaluated. This procedure is also used to compare several heat transfer coefficients in the refrigerant side. As in previous works the models of all the cycle components are considered together with the heat exchanger models in such a way that the system of equations they provide is solved by means of a Newton–Raphson algorithm. Calculated and measured values of the evaporation and the condensation temperatures are also compared. The experimental results correspond to the same air-to-water heat pump studied in other papers and they have been obtained by using refrigerants R-22 and R-290.     

Solid fraction modelling for CO2 and CO2–THF hydrate slurries used as secondary refrigerants

In the present paper, the suitability of hydrate slurries in secondary refrigeration was investigated by the means of a new hydrate solid-fraction model. Considering the high melting enthalpy of CO₂-containing hydrates, slurries presenting high hydrate solid fractions can carry sufficient latent heat to be useful for a two-phase secondary-refrigerant application. The model presented in this paper allowed to calculate the solid fraction of CO₂ and CO₂–THF hydrate from thermodynamic conditions of pressure and temperature. Contrary to a previous work on single CO₂ hydrates in a closed system, the present model can take into account hydrate mixture and is well adapted to additional CO₂ injections (opened system). By relying on the hydrate-conversion model results, the study of hydrates in suspension in a carrying liquid was also studied in an experimental loop and was based on a formation process by CO₂ injection in a cooled aqueous solution.     

Fundamental study on adhesion of ice to cooling solid surface

Many technological troubles are caused by ice adhesion to a cooling solid wall (surface). Therefore, it is urgent to clarify a mechanism of ice adhesion.It is thought that ice adhesion to the cooling wall is governed by heat transfer and interfacial phenomena between ice and the wall. In this study, shearing stress corresponding to adhesion force per unit area to remove ice from the wall surface and some reagents' contact angles on the wall and ice were measured, varying the wall material and its surface state. Moreover, shearing work to remove ice from the wall surface and surface energies of the wall and ice were calculated by the shearing stress and contact angles, respectively. And adhesion energy at an interface between ice and the wall was also calculated by the calculated surface energies. And then, influence of heat transfer and interfacial phenomena on ice adhesion was discussed to clarify the mechanism of ice adhesion.     

家用空调制冷系统清洁度浅析

本文主要就家用空调制造过程中空气、水分或其他常见杂质进入空调系统后对系统可能造成的影响进行讨论并给出相应的解决措施。     

NEW FOURIER–REYNOLDS CORRELATIONS FOR REFRIGERATION APPLICATIONS

In this paper, the development of new Fourier–Reynolds correlations for determining the cooling process parameters of products subject to air cooling, in terms of the half cooling times and seven-eighths cooling times is presented. The experimental data were taken from the author's previous work, processed and used in the development of new Fourier–Reynolds correlations and diagrams. These Fourier–Reynolds correlations are Fo_H=42·465Re^−0·54426 and Fo_S=125·21Re^−0·53913for half cooling times and seven-eighths cooling times, respectively. The results of this study indicate that there is a strong relationship between the Fourier and Reynolds numbers, and the Fourier–Reynolds correlations and diagrams are simple and effective tools for determining the cooling process parameters for such objects cooled in air flows. It is believed that these could be beneficial to the refrigeration industry. © 1997 by John Wiley & Sons, Ltd.     

A comprehensive design and rating study of evaporative coolers and condensers. Part I. Performance evaluation

The mathematical models of evaporative fluid coolers and evaporative condensers are studied in detail to perform a comprehensive design and rating analysis. The mathematical models are validated using experimental as well as numerical data reported in the literature. These models are integrated with the fouling model presented in an earlier paper, using the experimental data on tube fouling. In this paper, we use the fouling model to investigate the risk based thermal performance of these evaporative heat exchangers. It is demonstrated that thermal effectiveness of the evaporative heat exchangers degrades significantly with time indicating that, for a low risk level (p=0.01), there is about 66.7% decrease in effectiveness for the given fouling model. Furthermore, it is noted that there is about 4.7% increase in outlet process fluid temperature of the evaporative fluid cooler. Also, a parametric study is performed to evaluate the effect of elevation and mass flow rate ratio on typical performance parameters such as effectiveness for rating calculations while surface area for design calculations.     

Dynamic behavior of a direct expansion evaporator under frosting condition. Part I. Distributed model

A general distributed model with two-phase flow for refrigerant coupled with a frost model is developed for studying the dynamic behavior of an evaporator. The equations are derived in non-steady-state manner for the refrigerant and a quasi-steady state model with permeation for the frost. The complex flow and geometry of the finned tube evaporator lead to uneven wall and air temperature distributions, which in turn affect the rate of frost growth and densification along the coil depth. Results include frost accumulation and its effect on energy transfer, air off-coil temperature, refrigerant liquid dry-out position and propagation of frost formation along the coil.