The article presents the comparison of the head losses under the boiling refrigerant flow in horizontal and vertical evaporators. The values of all the components of the hydraulic resistance, such as the friction losses, local resistance, flow acceleration and leveling head have been calculated, compared and analyzed. Graphic dependences of the pressure loss of the two-phase flow on the mixture density and the apparatus height, as well as the effect of hydraulic resistances on the changes in boiling temperature have been obtained. It has been stated that studying the effectiveness of vertical evaporators comprises two main components: defining heat transfer and hydraulic resistance in the course of working fluids boiling in the vertical pipes. The second part of the study is under consideration. Vertical evaporators are higher than horizontal ones, which leads to greater losses when overcoming the leveling head of a larger column of a two-phase flow. Other components of the hydraulic resistance in the horizontal and vertical apparatus are comparable, although the friction losses and local resistance losses in the vertical evaporators may be less. In a vertical apparatus the main losses appear due to overcoming the leveling head, the remaining losses are smaller than in the horizontal apparatus. Therefore, when designing vertical apparatuses, one must not use the ratio of the apparatus length to its diameter, which is recommended for the horizontal apparatuses (L/d ratio for horizontal and vertical is 6). The ratio for the vertical apparatus should be significantly less. The experiments performed on the air-water model proved that L/d ratio of vertical evaporators should be within 1.0 ÷ 2. More precise values of this parameter can be found after testing R410A refrigerant.
The paper describes small-capacity irrigation evaporators that improve the performance of a refrigeration unit, as they exclude the release of liquid freon into the compressor suction pipe under sharp increasing of heat load or during ship rolling. The relevance of studying heat transfer at freons boiling in a moving film has been proved. The results and analysis of experimental data on average heat transfer coefficients are presented. The graph shows the dependence of the average heat transfer coefficients on the heat flux density at various irrigation densities. There are presented the results of special experiments determining the effect of irrigation density on heat transfer. It has been stated that the effect of pressure or saturation temperature in the modes of evaporation and developed boiling manifests itself in different ways. With developed boiling, the beam pitch does not have a significant effect on heat transfer. The experiments were carried out on two stands: small-row and multi-row. The pipes were heated with an internal electric heater. It has been inferred that heat transfer in the film is more intense than in volume, therefore, smooth steel pipes can be used in irrigation evaporators instead of finned copper tubes, which are used in flooded devices. The boiling process in a film can be described by equations valid for a large volume, taking into account quantitative differences. The values of a constant coefficient and the criteria exponents are given; the similarity equation for the regime of developed bubble boiling of freons is derived. The calculated dependencies can be applied in evaluating the operation of irrigation evaporators of ship refrigeration units.
The article describes the experimental evaporator as a research object, the evaporator model being given with boiling refrigerant R4O6A inside the pipe. The experiment in heat transfer was performed on a special stand inside a smooth pipe made of 1X18H9T grade steel with a diameter of 0.013 mm, a length of 3.3 m, and a wall thickness of 0.5 mm. There has been studied the influence of the two-phase regimes of the working medium movement on heat exchange where it is boiling both along the length of the pipe and in its section. The comparison between heat transfer coefficients has been given. The visual observations over the flow structure were held. There were determined four regimes of two-phase flow: emulsion, slug, wavy, and stratified. Changing heat transfer coefficients in the upper, medium and bottom parts of the tube section has been shown. Changing of heat transfer was identified in the dependence of developing of the regimes. There has been proved the existence of dry wall in the upper part of the pipe under wave and stratified regimes, where the minimal heat transfer was recorded. Graphic dependences of changing the heat transfer coefficients in the pipe section for various regimes are presented. It has been stated that the intensity of heat transfer to steam depends on the speed of its movement, and the heat transfer to a boiling liquid depends on two components: boiling intensity and speed of the liquid movement. It has been inferred that the refrigerant enters the evaporators of the ship refrigeration units after throttling with initial mass of steam content from 0.1 to 0.15 kg/kg, which corresponds to the projectile or wave mode; emulsion mode is possible only in pumping circuits; the length of the stratified regime with low heat transfer intensity will be the most significant.
The paper presents the results of an experimental study of the effect of oil on the heat transfer rate at boiling of mixed refrigerant R406A. Since the air conditioning system is not a pure refrigerant, but a mixture of oil with a concentration of up to 8%, such an amount of oil affects both hydrodynamics and heat exchange in the evaporators. The experimental work covers the entire range of regime parameters typical for these systems. There is shown the process of changing oil concentration in the pipe, as the working fluid boils, proving that most of the oil pipe does not impair the heat exchange in the course of two-phase flow boiling. Different modes of refrigerant R406A boiling dynamics have been defined, and each mode is given a quantitative assessment in terms of the effects of the oil and explaining of this effect on the fluid flow and heat transfer based on visual observations and the experiment results. The main factor of the effect is the freon-oil foam, which increases the proportion of the wetted surface in the wave and stratified modes and the heat transfer rate to 30%. A comparison of the heat transfer coefficients both in the cross section and along the pipe length has been performed, showing that the maximum change in heat transfer occurs in the upper part of the surface due to developing a dry wall on it and wetting it with freon-oil foam. A comparison of the heat transfer rate of pure refrigerant R406A has been done; the presence of oil in it shows that the effect of oil is complex and ambiguous. Calculation and criterion dependences for calculation of heat transfer coefficients in different modes have been proposed.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.