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The pre-swirl system is of great importance for temperature drop and cooling air supply. This study aims to investigate the influencing mechanism of heat transfer, nonuniform thermodynamic characteristics, and cooling air supply sensitivity in a pre-swirl system by the application of the flow control method of the pre-swirl nozzle. A novel test rig was proposed to actively control the supplied cooling air mass flow rate by three adjustable pre-swirl nozzles. Then, the transient problem of the pre-swirl system was numerically conducted by comparison with 60°, 120°, and 180° rotating disk cavity cases, which were verified with the experiment results. Results show that the partial nozzle closure will aggravate the fluctuation of air supply mass flow rate and temperature. When three parts of nozzles are closed evenly at 120° in the circumferential direction, the maximum value of the nonuniformity coefficient of air supply mass flow rate changes to 3.1% and that of temperature changes to 0.25%. When six parts of nozzles are closed evenly at 60° in the circumferential direction, the maximum nonuniformity coefficient of air supply mass flow rate changes to 1.4% and that of temperature changes to 0.20%. However, different partial nozzle closure modes have little effect on the average air supply parameters. Closing 14.3% of the nozzle area will reduce the air supply mass flow rate by 9.9% and the average air supply temperature by about 1 K. [ABSTRACT FROM AUTHOR]

Nonuniform performance of the parallel channels of the microchannel heat sink is one of the major limitations aggravating many undesired influences in both the single phase and the two phase heat transfer in microchannel heat sink. The nonuniform performance of the channels is preliminarily governed by the nonuniformity of the mass flow rate and heat flux transferred from the interface of parallel microchannels. The poor heat transfer (adiabatic/natural convection) boundary condition is primarily responsible for heat flux nonuniformity. In the current study a novel design of parallel microchannels heat sink is proposed, which is obtained by decreasing nonuniformity of mass flow rate and heat transfer existing between parallel channels of the microchannel heat sink. The nonuniformity of the mass flow rate is removed by the existing variable width approach and heat transfer nonuniformity among parallel channels is solved by the newly developed slanted microchannel provision. Developed design normalizes flow nonuniformity by 95.5% and heat transfer nonuniformity by 97.5% in comparison to the conventional design microchannel heat sink. The two major benefits offered by the proposed design are cooled (4.2 K lower than the conventional design) and uniform (3.1 K lesser than the conventional design) base surface temperature. It is also found that proposed work even facilitated in reduction of average base temperature (1.3 K lower than the conventional design). The maximum improvement in Nusselt number is 4.07%, also the proposed design extends benefits at off-design conditions even. [ABSTRACT FROM AUTHOR]

The uniformity degree of the air distribution over the heat-exchange surface of an air cooled condenser (ACC) plays an important role since it determines the efficiency of this surface's usage. In this paper, this factor at a special stand, where all the main processes of an air flow were simulated, is studied. The typical design of the A-framed ACC was chosen as the basis for the study. Two ACC models differing from one another by the presence of a diffuser after the fan were studied. Each of the models was tested in two versions: with a low-speed electric motor and a high-speed electric motor with a gearbox. In the ACC models, heat transfer surfaces were simulated by a set of flat metal grids with a uniform distribution of square cells. The aerodynamic drag of a set of grids was preliminarily selected so that the Euler criteria were equal for the full-scale object and model. During the tests, the air velocities in front of and behind the grids were measured by traversing, and the velocity fields (isotachs) of the cooling air in these sections were plotted. A condenser with a low-speed electric motor provides the most uniform distribution of cooling air over the heat-exchange surface, while condensers with a high-speed electric motor and gearbox and with a diffuser have significantly greater nonuniformity in the air supply. During the "suction" operation, the flow distribution is more uniform but worse than that for the version with a low-speed electric motor. The effect of the nonuniform velocity field of cooling air on the heat transfer of ACC is estimated. The real condenser module was calculated for various flow and velocity distributions along the tube length. Losses of heat removal due to the nonuniformity of the air flow of the considered circuits can be estimated within 1–3% at a ±50% deviation of the flow rate from the average value. [ABSTRACT FROM AUTHOR]

To investigate the effect of liquid distributing nonuniformity on the heat transfer of horizontal-tube falling film, a horizontal-tube falling-film air-cooled water chiller with an ejector liquid recirculation system (LRS) is presented, and then the finite difference model of the horizontal-tube falling-film evaporator is given. Also a new method of dividing the liquid refrigerant distributor into several subzones is proposed as well as the model of liquid refrigerant distributing nonuniformity is established. The analysis results show that increasing the liquid spraying flowrate in a certain range is obviously valid for enhancing the evaporator capacity, and the suitable R l is about 1.2, the corresponding value of φ is about 0.85. The analysis results also suggest that the higher the liquid recirculating ratio, the more the turning point of nonuniformity coefficient, and the turning point of nonuniformity coefficient is 0.1483 when the liquid recirculating ratio is equal to 1.21. The performance of falling-film heat transfer is more sensitive to the number of subzones under the condition of higher nonuniformity coefficient, and a suggested optimal number of partitioned subzones is 4. [ABSTRACT FROM AUTHOR]

In this paper, the gas-side fluid flow distribution inside a bayonet tube heat exchanger with inner and outer fins is numerically studied. The heat exchanger is designed based on the traditional bayonet tube heat exchanger, where compact continuous plain fins and wavelike fins are mounted on the outside and inside surfaces of outer tubes, respectively, to enhance the heat transfer performance. However, gross flow maldistribution and large vortices are observed in the gas-side flow channel of baseline design. In order to improve the flow uniformity, three modified designs are proposed. Three vertical plates and two inclined plates are mounted on the inlet manifold for Model B. For the Model C, another six bending plates are mounted on the middle manifolds and three pairs of them are connected together. The Model D has a similar structure as Model C except for the two additional baffles. The results indicate that the flow distributions of Models C and D are much more uniform under different inlet Reynolds number, especially in the high inlet Reynolds number. Although the flow distribution of Model D is the best, its pressure drop is 2.6 times higher than that of Model C. Therefore, the design of Model C is the most optimized structure. Compared with the original design, the nonuniformity of Model C can be reduced by 42% while the pressure drop is almost the same under the baseline condition. [ABSTRACT FROM AUTHOR]

Abstract: The transient temperature response of a crossflow heat exchanger is carried out using finite difference method accounting for the effect of temperature and flow nonuniformity at different input conditions. Beta flow maldistribution model has been introduced for the flow nonuniformity. The responses are found dependent on the relative position of the individual temperature streams and the position of the fluid moving device for the temperature and flow nonuniformity, respectively. Combined effect of temperature and flow nonuniformity has also been analysed and compared with the other cases. [Copyright &y& Elsevier]

The energy absorption capability is a key parameter for ZnO varistors. This paper discusses the nonuniformity of energy absorption capabilities of ZnO varistors. With experiment results, we show that energy absorption capabilities of ZnO varistors can be highly scattered, for different commercial varistors, and for the same varistors under diverse current waves. We explain this nonuniformity phenomenon through numerical simulations, based on the microscopic circuit and heat transfer models of ZnO varistors. How the applied current, the nonuniformity of microstructure, and the nonuniformity of microstructural electrical and thermophysical characteristics influence the energy absorption capabilities of ZnO varistors is also analyzed. [ABSTRACT FROM AUTHOR]