Интенсификация теплопередачи в двухфазных системах с капиллярными насосами

Heat transfer in capillary pumped loops (CPL) is carried out by transferring the mass of the circulating coolant in the form of liquid and vapor. Therefore, the hydrodynamics of the phases in the CPL determines their heat transfer capacity (heat flow or the product of the heat flow by the heat transfer length). The influence of structural, hydraulic and thermo-physical properties of capillary structures used as capillary pumps in two-phase thermal control systems (Loop Heat Pipes - LHP) on their heat transfer capacity has been analyzed. Methods of increasing the heat transfer capacity of LHP, due to the use of anisotropic capillary structures with a decrease in pore sizes in the direction of the vaporization zone, have been determined. The conditions of LHP operability and the method of analytical calculation of the temperature field in anisotropic capillary structures for a model with pseudo-convection have been considered. The calculated and experimental data have been compared.

Numerical Simulation of Heat Transfer Tube Characteristics of Refrigeration and Air Conditioning Based on Mixed Fractional Model

The main parameters affecting the heat transfer performance of heat transfer tube heat exchanges include fin shape, fin spacing, fin thickness, tube row arrangement, tube diameter, dry and wet bulb temperature and flow rate. The air side heat transfer performance of heat transfer tube heat exchange and the influence of velocity field and temperature field distribution on heat transfer effect have been the focus of domestic and foreign scholars. In this paper, based on the mixed fraction model, CFD software is used to simulate the absorption process of gravity falling film outside the heat transfer tubes of refrigeration and air conditioning, and to study the flow and heat transfer characteristics of the process. The results show that, for the heat transfer tubes with the selected structure, the heat transfer capacity increases with the increase of water flow velocity, and the heat transfer enhancement effect of turbulence is enhanced. The heat transfer tubes have better comprehensive heat transfer performance than smooth tubes with the same diameter.

THE VERIFICATION OF THE RSG-GAS REACTOR COOLING TOWER HEAT TRANSFER CAPACITY

The RSG-GAS reactor has been replaced and the technical specifications for the new cooling tower specify that the heat transfer capacity from the secondary cooling water to the environment is 5500 kW per module. Therefore, this study aims to verify the theoretical calculations of the heat transfer capacity using performance test data collected on the 30 MW power operation on December 20, 2018, such as the temperature of the primary and secondary coolant entering and exiting the cooling tower, wet bulb, and environmental dry bulb temperature, as well as the inlet and outlet air temperature. Furthermore, the data were used to calculate the heat transfer capacity from the secondary cooling water to the environment. The results showed that each cell of the RSG-GAS cooling tower reactor transfers the heat of approximately 5528.52 kW. This value is consistent with the technical specifications written in the revised RSG-GAS Safety Analysis Report 11.

Experimental investigation of a loop heat pipe with R245fa and R1234ze (E) as working fluids

Two kinds of new refrigerant-R1234ze (E) and R245fa were discussed as substitutes or supplements to traditional working fluids of loop heat pipes based on their favorable thermophysical properties and characteristics such as being safe and environmentally friendly. Thermal characteristics of a loop heat pipe with sintering copper wick at different charging ratios were experimentally investigated under variable heat loads. The results showed that the optimal charging ratio in the loop heat pipe range from 65% to 70%, and at this charging level, the R1234ze(E) system had better start-up response, while the R245fa system presented a stronger heat transfer capacity. The characteristic temperature of R1234ze(E) system was below 35 °C, and the corresponding thermal resistance was 0.08 K/W ~ 1.62 K/W under heat loads ranging from 5 W to 40 W. The thermal resistance of the R245fa system was 0.18 K/W ~ 0.91 K/W under heat loads of 10 W ~ 60 W, and the operating temperature was below 60 °C. The loop heat pipes charged with the proposed new refrigerants exhibit superb performance in room temperature applications, making them beneficial for enhancing the performance of electronics, and could provide a distinctive choice for the cooling of small-sized electronics especially.

Research on heat transfer characteristics and typical parameters sensitivity analysis of the deep borehole heat exchanger combined with the geothermal wells

Based on abundant hydrothermal geothermal resources at the depth of 1000-2000m formation in the basin of the BoHai Bay, the deep borehole heat exchanger (DBHE) combined with the geothermal wells is proposed. According to the modified thermal resistance and capacity model (MTRCM), the heat transfer models inside and outside borehole are established. The transient analytical solutions are obtained by applying Laplace transform method to calculate the vertical temperature profiles in the inlet (outlet) pipe and the grout of the DBHE. The mathematical model and the analytical solutions are validated by the experimental data and existing studied data. This paper utilizes respectively the Matlab2012 and the Feflow7.1 to solve the heat transfer models inside and outside the DBHE. The sensitivity analysis is performed to examine the influence of typical parameters on the DBHE heat transfer characteristics. Under the well distance of 50m, the DBHE heat transfer capacity increases by 29.5% and 42.5% when the quantity of geothermal water exploitation increases from 0m3/h to 75m3/h and 150m3/h respectively. The results show that the heat transfer mechanism is changed in the thermal reservoir, and the heat transfer progress of the DBHE is intensified through orderly regulating the quantity of geothermal water exploitation and the well distance. However, with the change of the quantity of geothermal water exploitation, the growth rate of the DBHE heat transfer capacity reduces and the sensitivity of the change of the typical parameters on the DBHE heat transfer performance reduces.

Numerical Study on the Fluid Flow and Heat Transfer Characteristics of Al2O3-Water Nanofluids in Microchannels of Different Aspect Ratio

The study of the influence of the nanoparticle volume fraction and aspect ratio of microchannels on the fluid flow and heat transfer characteristics of nanofluids in microchannels is important in the optimal design of heat dissipation systems with high heat flux. In this work, the computational fluid dynamics method was adopted to simulate the flow and heat transfer characteristics of two types of water-Al2O3 nanofluids with two different volume fractions and five types of microchannel heat sinks with different aspect ratios. Results showed that increasing the nanoparticle volume fraction reduced the average temperature of the heat transfer interface and thereby improved the heat transfer capacity of the nanofluids. Meanwhile, the increase of the nanoparticle volume fraction led to a considerable increase in the pumping power of the system. Increasing the aspect ratio of the microchannel effectively improved the heat transfer capacity of the heat sink. Moreover, increasing the aspect ratio effectively reduced the average temperature of the heating surface of the heat sink without significantly increasing the flow resistance loss. When the aspect ratio exceeded 30, the heat transfer coefficient did not increase with the increase of the aspect ratio. The results of this work may offer guiding significance for the optimal design of high heat flux microchannel heat sinks.

Numerical Study on the Fluid Flow and Heat Transfer Characteristics of Al2O3-water Nanofluids in Microchannels of Different Aspect Ratio

The study of the influence of the nanoparticle volume fraction and aspect ratio of microchannels on the fluid flow and heat transfer characteristics of nanofluids in microchannels is important in the optimal design of heat dissipation systems with high heat flux. In this work, the computational fluid dynamics method was adopted to simulate the flow and heat transfer characteristics of two types of water–Al2O3 nanofluids with two different volume fractions and five types of microchannel heat sinks with different aspect ratios. Results showed that increasing the nanoparticle volume fraction reduced the average temperature of the liquid–solid heat transfer surface and thereby improved the heat transfer capacity of the nanofluids. Meanwhile, the increase of the nanoparticle volume fraction led to a considerable increase in the pumping power of the system. Changing the aspect ratio of the microchannel effectively improved the heat transfer capacity of the heat sink. Moreover, increasing the aspect ratio effectively reduced the average temperature of the heating surface of the heat sink without significantly increasing the flow resistance loss. When the aspect ratio exceeded 30, the heat transfer coefficient did not increase with the increase of the aspect ratio. The results of this work may offer guiding significance for the optimal design of high heat flux microchannel heat sinks.

Research of the process of obtaining capillary-porous materials from metal powders for heat pipes

Heat pipes are designed to effective removing heat from heating elements and reducing the temperature of various devices. Heat pipes with capillary porous structures are designed to operate under conditions of unfavorable gravity forces. Their main advantages are their high heat transfer capacity, as well as the ability to retain the coolant in a capillary-porous structure under dynamic power loads. The purpose of this work is to study the process of obtaining capillary-porous materials from metal powders for heat pipes with increased efficiency of using the vibration molding method. The article substantiates the relevance of creating heat pipes from metal powders. The information about the influence of the contact angle, surface tension and capillary pressure on the heat transfer capacity of a heat pipe is provided. It is shown that for the efficient operation of the heat pipe it is necessary to create such a capillary structure of the porous material, which could simultaneously provide a high speed of movement of the coolant and its rise to a given height. The above requirements can be satisfied by creating a capillary structure using powder metallurgy methods by optimizing the distribution of pore sizes. In this case, the most promising method seems to be the method of molding when applying a vibration to a mold with a powder. It is possible to obtain the required pore distribution in this way by choosing the correct particle size, shape and vibration parameters. This makes it possible to ensure the packing of particles in size, which affects their packing density, pore size, tortuosity and length of pore channels. The distribution of the maximum pore sizes over the thickness of the samples obtained from powders of various granulometric composition with the use of vibration has been investigated. As a result, a process was developed for obtaining capillary structures by the method of vibration molding of metal powders, depending on the size of the powder particles, the amplitude and frequency of vibration. It is shown that this method can provide a given pore distribution of the capillary structure for heat pipes, which makes it possible to increase their heat transfer capacity.