Flow mal-distribution of refrigerant in small diameter tube heat exchangers is a great concern, which may lead to a 25% efficiency loss. Two-phase refrigerant distributors are set before evaporators to separate refrigerant into parallel paths uniformly. In this paper, a new type of distributor with two barrels is proposed. Experimental test and numerical simulation were both carried out to evaluate the performance and to understand internal hydrodynamic flow behavior. Compared with the previous distributor, it is found that the double-barrel distributor with proper parameters performs better. The relative error between experimental and simulation results is less than 15%, which proves the reliability of the established simulation model. Computation fluid dynamics (CFD) calculation indicates that the distribution performance is improved with properly larger bottom and top barrel diameters. With the increase of the bottom barrel diameter, beneficial reflux of refrigerant occurs in bottom barrel and when top barrel diameter is larger, little refrigerant flows directly into outlet capillary tubes without mixture or reflux. In addition, parameters such as top barrel diameter, top barrel height, bottom barrel diameter, bottom barrel height, mass flow rate and quality are studied by Taguchi Method to analyze the parameter sensitivity. The effect of the parameters listed below ranges from biggest to smallest: mass flow rate, bottom barrel height, quality, top barrel height, bottom barrel diameter and independent top barrel diameter. An optimized two-barrel distributor is achieved with proper top and bottom barrel diameters and larger bottom and top barrel heights.
By means of experiment and simulation, we investigated the coupling between the accumulator and the loop in the Tracker Thermal Control System (TTCS) system, which is a typical mechanically pumped carbon dioxide two-phase loop, by studying the system behaviors under the disturbance of the temperature boundary of the condensers, and the step change of the evaporator heat load. We found that the disturbance of the loop affects the accumulator, and in turns, affects the loop itself. If the heat compensation of the
Stability is a key factor that limits the application of liquid-vapor two-phase loop. in this paper, we investigated the two-phase flow stability boundaries of two evaporators in parallel in a mechanically pumped CO 2 two-phase loop(MPTL), which distinguish steady flow, flow oscillations at the inlet, and temperature oscillations at the outlets of the evaporators. We inferred that the instability is the result of density wave oscillation (DWO), and found that the periods of the flow oscillations are comparable with the residence time of CO 2 fluid particle in the evaporator.
We investigated experimentally the start-up characteristics of a mechanically pumped two-phase loop (MPTCL), with CO2 as working fluid, and a single evaporator that consists of a bent inner ring and an outer ring constructed by stainless tubes with hydraulic diameter of 2.6 mm and length of 9 m, along which totally 54 pieces of heating element are distributed. Experiments were performed in the following conditions: mass flow rates of 1.1, 2.1, and 3.3g/s; heat loads ranged from 50 to 300W, with the heat-load ratios of the inner ring to the outer ring 2.2:1, 1:1, and 1:2.2 at the operational temperature of −15°C, respectively. During the start-up cases, we detected a reverse flow accompanying with pressure spike, which can be understood as explosive boiling, and a subsequent temporal dry-out phenomenon at the outlet of the evaporator, as a result of explosive boiling. The back flow together with the pressure spike is helpful to set up a two-phase flow all along the evaporator, though it may have negative effect on the loop, especially, when coincident explosive boiling happens. However, such a pressure spike that depends on initial superheating should be controlled to avoid possible harm to the loop.
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