Selected Problems of Experimental Investigations during Refrigerants Condensation in Minichannels
Tadeusz Bohdal, Henryk Charun, and Malgorzata Sikora
Koszalin University of Technology
Introduction
The dynamic development of technologies in space technology and electronics has an impact on the global trend of machines and devices miniaturization in many areas. In some systems, there is a need to discharge a high heat flux density, larger than 1,000 W/cm2 (Baummer et al. 2008). The use of conventional methods of heat collection has become ineffective or even impossible. In this case, the phase changes of refrigerants (condensation and boiling) are used, especially in small-diameter channels (Obhan and Garimella 2001). High values of heat transfer coefficient are obtained; they allow reducing the heat exchange area. Practical implementation of this method is done using compact minicondensers and minievaporators. To call the heat exchanger compact, it should be made from channels with a hydraulic diameter (with dimensions depending on the criteria) (Ohadi 2007, Mehendale et al. 2000, Kandlikar 2003) within 1-6 mm, usually in practice dh < 3 mm.
In recent years, the number of articles about research methods and theoretical analysis in terms of boiling and condensation in small-diameter channels has significantly increased. The number of publications in the boiling range in minichannels is also higher. However, it is not possible to transfer in absolute terms the test results obtained during boiling in minichannel for the condensation process in this type of channels (Mikielewicz 1995, Thome et al. 2003a,b, Cavallini et al. 2005). A significant difficulty in designing and testing of compact minicondensers is the fact that there are no uniform models for the condensation of refrigerants (Sun and Mishima 2009, Garcia-Cascales et al. 2010). This chapter focuses on research problems related to experimental investigations of heat exchange and flow resistance during condensation of refrigerants in minichannels and also two-phase flow structures.
Some Research Problems of Compact Minicondensers
While designing compact refrigeration heat exchangers, two basic engineering problems should be solved. Firstly, determine, for a selected heat exchanger, the heat exchange surface and the driving power of the motion generators of the refrigerant that transmits the heat. This is usually done to calculate the values of heat transfer coefficient on both sides of the wall, during boiling or condensation of refrigerants in the flow and for factors mediating in heat transfer (water, air, brine, etc.). The driving power is determined on the basis of the knowledge of the mass flow value and the flow resistance of the refrigerant and intermediary factor. For relatively simple cases, the best known calculation correlations from the literature are used.
An effective way to determine the thermal and flow characteristics of boiling and condensation in minichannels is experimental research. In boiling tests in minichannels, the heat flow from the environment is directly supplied to the measuring section of the minichannel (often using electric heating). In the refrigerant condensation research in the minichannel, the heat flow must be transferred to the cooling medium, which is unfortunately a much more difficult problem. Therefore, indirect methods of collecting heat flux from the test section are usually used.
The Mechanism of Refrigeration Condensation in Minichannels and Basic Research Problems
The refrigerant condensation process in a compact minicondenser is similar to that in a conventional one. The superheated refrigerant vapor, after leaving the compressor discharge port, is directed to the condenser. In the first zone of condenser, the heat of the superheated steam is collected, and when the wall temperature of the minichannel reaches a value lower than the saturation temperature, the process of proper condensation begins. This process continues to achieve the saturated liquid state for the vapor quality x = 0. Condensation may be complete (in the range of vapor quality from x = 1 to x = 0) or incomplete. After the proper condensation zone, a single-phase process of the refrigerant liquid subcooling is carried out. It has experimentally been found for a compact condenser (Leducq et al. 2003) that the heat transfer zone for superheated steam can account for up to 15% of the total surface area of the exchanger, the area of the second zone (proper condensation) is 73%-80%, and that of the subcooling zone is 5%-12% of the heat exchanger area.
In both the conventional and compact condensers, the most important effect on heat exchange efficiency is due to the proper condensation zone, in which the heat transfer coefficient reaches high values, compared to single-phase zones of superheat and subcooling heat recovery. In the case of homogeneous refrigerant condensation, the isothermal and isobaric nature of the process is preserved, but during the condensation of zeotropic mixtures, the process is non-isothermic and proceeds with a temperature glide. For the condensation process in the conventional channel, the dominant influence is from gravity, inertial forces, and shear forces at the interface, causing changes in the two-phase flow structure.
The mechanism of energy and momentum exchange during refrigerants condensation in minichannels is more complicated than in convention channels. The interactions of viscous forces and surface tension play a fundamental role in it. This leads to the formation other two-phase flow structures than those in the conventional channels.
The basic experimental research problems of refrigerants condensation in minichannels are as follows: [1] [2] [3] [4] [5] [6]
Some of the mentioned research problems are presented in the further part of the chapter. The importance of the research problem of the refrigerants condensation process in small-diameter channels is the reason for widely developed research cooperation of many science centers in this topic. An example is the cooperation of the University of Padova (Cavallini et al. 2005) and the University of Pretoria (Ewim and Meyer 2019, Ewim et al. 2018) in the field of condensation of new refrigerants, including zeotropic mixtures (Thome 2005).
Methodology of Experimental Investigations of Refrigerants Condensation in Minichannels
While developing the methodology of experimental research on condensation in mini-channels, modern research trends should be taken into account. Trends relate to the use of high-accuracy measuring devices. For example, in their study, Thome et al. (2003b) showed that in condensation investigations in the 1990s, Coriolis flow meters with a great accuracy were used. The condensation tests in channels were very difficult when the vapor quality is v < 0.05 or ,t > 0.95. Also in the literature, only few publications regarding condensation in minichannels, for mass flux density G < 50 kg/ms exist. Below this value, there are big problems with measuring accuracy.
Some parameters of condensation characteristics in minichannels can be measured directly (e.g., refrigerant, intermediate factor, or channel wall temperature, pressure, and differential pressure). Other parameters are determined by indirect methods (including heat flux from the measurement section, vapor quality x, void fraction e, and average and local heat transfer coefficients a). One of the intermediate research methods is the Wilson method (Wilson 1915), with its later modifications, but its range is limited.
In most cases, the experimental test stand for the condensation of homogeneous refrigerants (pure refrigerant) can also be used in studies of mixtures of these refrigerants, including zeotropic mixtures (Macdonald and Garimella 2016).
- [1] Assessment of the mass, energy, and momentum exchange mechanisms during the condensation of refrigerants in minichannels.
- [2] Development of the maps of two-phase flow structures in a generalized version, especially for new pro-ecological refrigerants.
- [3] Determining the influence of the refrigerant type on the parameters of the condensation process and the results of thermal and flowinvestigations.
- [4] Determination of the influence of minichannel geometric parameters (including internal diameter, shape and length) on the condensation efficiency.
- [5] Indication of generalized and experimentally tested procedures forcalculating the average and local values of heat transfer coefficientand flow resistance.
- [6] Development of theoretical basis for exchange and momentumenergy for condensation in minichannels, with simultaneous explanation of differences in process mechanisms in conventional, mini-,and microchannels.
