scholarly journals Grieb M., Brümmer A. Investigation into the effects of surface condensation in steam-driven twin screw expanders / trans. from Engl. M. A. Fedorova

2021 ◽  
Vol 5 (1) ◽  
pp. 44-52
Author(s):  
Keyword(s):  
Model Simulation ◽  
Thermodynamic Simulation ◽  
Working Fluid ◽  
Condensation Process ◽  
Transfer Coefficients ◽  
Two Phase ◽  
Surface Condensation ◽  
Twin Screw ◽  
Chamber Model ◽  
Machine Parts

During the operation of twin screw expanders with slightly superheated vapours or even two-phase fluids, surface condensation on machine parts occurs during the filling period and the expansion phase when the working fluid is in contact with cooler inner surfaces. This heat exchange from the working fluid to adjacent machine parts effects the working cycle and the efficiency of these machines. Short time scales and the periodicity of the process indicate the condensation process is best described by models for dropwise condensation. In this paper the effects of surface condensation on the operation of twin screw expanders are initially discussed in a simulation-based investigation. Chamber model simulation coupled with a thermal analysis is used for the thermodynamic simulation, whereby heat transfer coefficients are systematically varied. It is found that during the inlet phase condensate emerges on the inner surfaces of the machine being substantially cooler than the working fluid. This results in a higher mass being trapped within the working chamber and, thus, an increasing mass flow rate of the machine. An increase in power output is, however, not observed. The results obtained from chamber model simulations are finally compared against experimental data of a screw expander prototype

Converting Low-Grade Heat Into Power Using a Supercritical Rankine Cycle With Zeotropic Mixture Working Fluid

2010 ◽  
Author(s):  
Huijuan Chen ◽  
D. Yogi Goswami ◽  
Muhammad M. Rahman ◽  
Elias K. Stefanakos
Keyword(s):  
Rankine Cycle ◽  
Phase Region ◽  
Working Fluid ◽  
Condensation Process ◽  
Heating Process ◽  
Low Grade ◽  
Two Phase ◽  
Working Fluids ◽  
Zeotropic Mixture ◽  
Low Grade Heat

A supercritical Rankine cycle using zeotropic mixture working fluids for the conversion of low-grade heat into power is proposed and analyzed in this paper. A supercritical Rankine cycle does not go through two-phase region during the heating process. By adopting zeotropic mixtures as the working fluids, the condensation process happens non-isothermally. Both of the features create a potential in reducing the irreversibility and improving the system efficiency. A comparative study between an organic Rankine cycle and the proposed supercritical Rankine cycle shows that the proposed cycle improves the cycle thermal efficiency, exergy efficiency of the heating and the condensation processes, and the system overall efficiency.

scholarly journals EFFECT OF OIL ON HEAT TRANSFER OF MIXED REFRIGERANT BOILING IN EVAPORATOR TUBES IN REFRIGERATION MACNINES

2019 ◽  
pp. 88-93
Author(s):  
Alexey Vasilievich Ezhov ◽  
Sergey Sergeevich Ivanov ◽  
Aleksandr Bukin ◽  
Vladimir Grigorievich Bukin
Keyword(s):  
Heat Transfer ◽  
Heat Exchange ◽  
Heat Transfer Rate ◽  
Transfer Rate ◽  
Flow Boiling ◽  
Working Fluid ◽  
Transfer Coefficients ◽  
Two Phase ◽  
Pipe Length ◽  
Heat Transfer Coefficients

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.

A Distributed Model for Air-to-Refrigerant Fin-and-Tube Evaporators With Special Emphasis on Two-Phase Zone

2010 ◽  
Author(s):  
Haiyan Li ◽  
Lige Tong ◽  
Xinxing Sun ◽  
Li Wang ◽  
Shaowu Yin
Keyword(s):  
Heat Transfer ◽  
Cooling Capacity ◽  
Distributed Model ◽  
Working Fluid ◽  
Flow State ◽  
Transfer Coefficients ◽  
Phase Zone ◽  
Two Phase ◽  
Pressure Drops ◽  
Error Band

A general and simple model for simulating the steady behavior of air-to-refrigerant fin-and-tube evaporators, which accounts for detailed flow state inside the tubes, is introduced. To account for the heat transfer between air and the working fluid, the evaporator is divided into a number of control volumes. Space dependent partial differential equations group is obtained from the mass, energy and momentum balances for each one. The corresponding discretized governing equations are solved afterwards. Empirical correlations are also required to estimate the void fraction, the internal and external heat transfer coefficients, as well as the pressure drops. According to the phase of refrigerating fluid, the evaporator can be divided into two distinct zones on the refrigerant-side: the vapor zone and the two-phase zone, while special emphasis is performed on the treatment of the two-phase zone. The distribution of flow pattern has been evaluated with the aim of improving the calculation accuracy. The model prediction is validated against experimental data for an evaporator using R22 as the working fluid, which shows a reasonable level of agreement: the cooling capacity is predicted within the error band of 3%. The developed model will have wide applications in operational optimization, performance assessment and pipeline design.

Vibration Modeling and Experimental Results of Two-Phase Twin-Screw Pump

2016 ◽  
Vol 138 (9) ◽  
Author(s):  
Ameen Muhammed ◽  
Dara W. Childs
Keyword(s):  
Gas Turbines ◽  
Critical Speed ◽  
Two Phase Flow ◽  
Dynamic Pressure ◽  
Working Fluid ◽  
Phase Flow ◽  
Running Speed ◽  
Two Phase ◽  
Screw Pump ◽  
Twin Screw

In turbomachines, the transfer of energy between the rotor and the fluid does not—in theory—result in lateral forces on the rotor. In positive displacement machines, on the other hand, the transfer of energy between the moving components and the working fluid usually results in unbalanced pressure fields and forces. Muhammed and Childs (2013, “Rotordynamics of a Two-Phase Flow Twin Screw Pump,” ASME J. Eng. Gas Turbines Power, 135(6), p. 062502) developed a model to predict the dynamic forces in twin-screw pumps, showing that the helical screw shape generates hydraulic forces that oscillate at multiples of running speed. The work presented here attempts to validate the model of Muhammed and Childs (2013, “Rotordynamics of a Two-Phase Flow Twin Screw Pump,” ASME J. Eng. Gas Turbines Power, 135(6), p. 062502) using a clear-casing twin-screw pump. The pump runs in both single and multiphase conditions with exit pressure up to 300 kPa and a flow rate 0.6 l/s. The pump was instrumented with dynamic pressure probes across the axial length of the screw in two perpendicular directions to validate the dynamic model. Two proximity probes measured the dynamic rotor displacement at the outlet to validate the rotordynamics model and the hydrodynamic cyclic forces predicted by Muhammed and Childs (2013, “Rotordynamics of a Two-Phase Flow Twin Screw Pump,” ASME J. Eng. Gas Turbines Power, 135(6), p. 062502). The predictions were found to be in good agreement with the measurements. The amplitude of the dynamic pressure measurements in two perpendicular plans supported the main assumptions of the model (constant pressure inside the chambers and linear pressure drop across the screw lands). The predicted rotor orbits at the pump outlet in the middle of the rotor matched the experimental orbits closely. The spectrum of the response showed harmonics of the running speed as predicted by the model. The pump rotor's calculated critical speed was at 24.8 krpm, roughly 14 times the rotor's running speed of 1750 rpm. The measured and observed excitation frequencies extended out to nine times running speed, still well below the first critical speed. However, for longer twin-screw pumps running at higher speed, the coincidence of a higher-harmonic excitation frequency with the lightly damped first critical speed should be considered.

scholarly journals A Novel Generator Design Utilised for Conventional Ejector Refrigeration Systems

Energies ◽  
2021 ◽  
Vol 14 (22) ◽  
pp. 7705
Author(s):  
Anas F. A. Elbarghthi ◽  
Mohammad Yousef Hdaib ◽  
Václav Dvořák
Keyword(s):  
Heat Transfer ◽  
Heat Exchanger ◽  
Pressure Drop ◽  
Working Fluid ◽  
Convection Coefficient ◽  
Transfer Coefficients ◽  
Two Phase ◽  
Heat Transfer Coefficients ◽  
Cold Fluid ◽  
Cold Stream

Ejector refrigeration systems are rapidly evolving and are poised to become one of the most preferred cooling systems in the near future. CO2 transcritical refrigeration systems have inherently high working pressures and discharge temperatures, providing a large volumetric heating capacity. In the current research, the heat ejected from the CO2 gas cooler was proposed as a driving heating source for the compression ejector system, representing the energy supply for the generator in a combined cycle. The local design approach was investigated for the combined plate-type heat exchanger (PHE) via Matlab code integrated with the NIST real gas database. HFO refrigerants (1234ze(E) and 1234yf) were selected to serve as the cold fluid on the generator flowing through three different phases: subcooled liquid, a two-phase mixture, and superheated vapour. The study examines the following: the effectiveness, the heat transfer coefficients, and the pressure drop of the PHE working fluids under variable hot stream pressures, cold stream flow fluxes, and superheated temperatures. The integration revealed that the cold fluid mixture phase dominates the heat transfer coefficients and the pressure drop of the heat exchanger. By increasing the hot stream inlet pressure from 9 MPa to 12 MPa, the cold stream two-phase convection coefficient can be enhanced by 50% and 200% for R1234yf and R1234ze(E), respectively. Conversely, the cold stream two-phase convection coefficient dropped by 17% and 37% for R1234yf and R1234ze(E), respectively. The overall result supports utilising the ejected heat from the CO2 transcritical system, especially at high CO2 inlet pressures and low cold channel flow fluxes. Moreover, R1234ze(E) could be a more suitable working fluid because it possesses a lower pressure drop and bond number.

Parametric Dependence of Annular Flow Heat Transfer in Microgaps

2010 ◽  
Author(s):  
Emil Rahim ◽  
Avram Bar-Cohen
Keyword(s):  
Heat Transfer ◽  
Annular Flow ◽  
Forced Flow ◽  
Working Fluid ◽  
Transfer Coefficients ◽  
Two Phase ◽  
Channel Diameter ◽  
Parametric Dependence ◽  
Heat Transfer Coefficients ◽  
High Heat Transfer

Forced flow of refrigerants and dielectric liquids, undergoing phase change in a heated microgap channel between chips or in parallel microchannels in a compact cooler, is a promising candidate for the thermal management of advanced semiconductor devices. It has been found that Annular flow is the dominant flow regime in such miniature channels and that relatively high heat transfer coefficients are encountered in the moderate-to-high quality sections of such channels. Following a discussion of flow regimes and thermal characteristics of miniature channels, attention turns to exploring the parametric dependence of annular flow thermal transport in microgaps including the effects of channel diameter, mass flux, and working fluid on the two-phase heat transfer coefficients.

scholarly journals Numerical Simulation and Analyses of Flow Rate Exchange between Accumulator and Main loop

2019 ◽  
Vol 128 ◽  
pp. 04005
Author(s):  
Meng Qingliang ◽  
Zhao Zhenming ◽  
Zhang Huandong
Keyword(s):  
Numerical Model ◽  
Stokes Equations ◽  
Model Simulation ◽  
Navier Stokes ◽  
Working Fluid ◽  
Total System ◽  
Two Phase ◽  
Main Loop ◽  
Temperature And Pressure ◽  
Phase Fluid

In order to study the dynamic behaviors of heat and mass transfer between accumulator and mechanically pumped two–phase loop (MPTL) system, a transient numerical model is developed by using thetime–dependent Navier–Stokes equations. By comparison between simulation and test results, it is found that the error of numerical model is in the range of ±10%, which verifies the validityand accuracy of the model. Simulation results show that the accumulator will exchange fluid with the main loop in responseto heat load variations. In this case, the temperature and pressure of two phase fluid in accumulator, and the total system flow resistance will be affected. The rate of mass transferbetween accumulator and main loop will increase along with the charge amount of working fluid, and also for the variation trend of temperature and pressure of two phase fluid in the accumulator. The model can be used to study the operating state, flow and heat characteristics of MPTL system.

Boiling Heat Transfer From a Horizontal Tube in an Upward Flowing Two-Phase Crossflow

1984 ◽  
Vol 106 (4) ◽  
pp. 849-855 ◽  
Author(s):  
M. E. Wege ◽  
M. K. Jensen
Keyword(s):  
Heat Transfer ◽  
Vertical Channel ◽  
Horizontal Tube ◽  
Nucleate Boiling ◽  
Working Fluid ◽  
Transfer Coefficients ◽  
Two Phase ◽  
Average Deviation ◽  
Wall Superheat ◽  
Two Phase Heat Transfer

An experimental investigation has been performed to determine the effects of a low-quality (≤ 20 percent) upward flowing mixture on the nucleate boiling on a single horizontal lube. An electrically heated, 12.7-mm-dia tube was centered in a plane wall vertical channel, the width of which resulted in channel width-to-tube diameter ratios (w/d) of 1.16 and 1.95. The working fluid was R-113. The two-phase heat transfer data showed a variety of effects. For a fixed w/d, pressure (P), and quality (x), the average heat transfer coefficients (h) increased with increasing mass velocity (G), but the effect of G decreased as the wall superheat (ΔT) increased. For a fixed w/d, G and x, h increased as the pressure increased except at low ΔT’s where the reverse was found. For fixed w/d, P and G, h increased with increasing quality with the effect appearing to be more pronounced at the lower pressure. At a fixed P, G and x, h was at larger w/d ratios at small ΔT’s, but as the wall superheat increased an inversion occured and h became smaller at the larger w/d ratio. The behavior exhibited in this experiment can be explained in terms of the velocity of the fluid flowing past the test section. The data were successfully predicted to within an average deviation of ±11.6 percent using a Chen-type correlation. Data from the literature also were predicted well.

Thermal Modeling of Extreme Heat Flux Microchannel Coolers for GaN-on-SiC Semiconductor Devices

2016 ◽  
Vol 138 (1) ◽  
Author(s):  
Hyoungsoon Lee ◽  
Damena D. Agonafer ◽  
Yoonjin Won ◽  
Farzad Houshmand ◽  
Catherine Gorle ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Thermal Performance ◽  
Working Fluid ◽  
Extensive Literature ◽  
High Electron ◽  
Flow Conditions ◽  
Transfer Coefficients ◽  
Two Phase ◽  
Heat Transfer Coefficients ◽  
Power Densities

Gallium nitride (GaN) high-electron-mobility transistors (HEMTs) dissipate high power densities which generate hotspots and cause thermomechanical problems. Here, we propose and simulate GaN-based HEMT technologies that can remove power densities exceeding 30 kW/cm2 at relatively low mass flow rate and pressure drop. Thermal performance of the microcooler module is investigated by modeling both single- and two-phase flow conditions. A reduced-order modeling approach, based on an extensive literature review, is used to predict the appropriate range of heat transfer coefficients associated with the flow regimes for the flow conditions. Finite element simulations are performed to investigate the temperature distribution from GaN to parallel microchannels of the microcooler. Single- and two-phase conjugate computational fluid dynamics (CFD) simulations provide a lower bound of the total flow resistance in the microcooler as well as overall thermal resistance from GaN HEMT to working fluid. A parametric study is performed to optimize the thermal performance of the microcooler. The modeling results provide detailed flow conditions for the microcooler in order to investigate the required range of heat transfer coefficients for removal of heat fluxes up to 30 kW/cm2 and a junction temperature maintained below 250 °C. The detailed modeling results include local temperature and velocity fields in the microcooler module, which can help in identifying the approximate locations of the maximum velocity and recirculation regions that are susceptible to dryout conditions.

Choosing Working Fluid for Two-Phase Thermosyphon Systems for Cooling of Electronics

2003 ◽  
Vol 125 (2) ◽  
pp. 276-281 ◽  
Author(s):  
Bjo¨rn Palm ◽  
Rahmatollah Khodabandeh
Keyword(s):  
Heat Transfer ◽  
High Pressure ◽  
Heat Fluxes ◽  
Working Fluid ◽  
Air Cooling ◽  
Transfer Coefficients ◽  
Two Phase ◽  
Heat Transfer Coefficients ◽  
And Performance ◽  
Cooling Of Electronics

The heat fluxes from electronic components are steadily increasing and have now, in some applications, reached levels where air-cooling is no longer sufficient. One alternative solution, which has received much attention during the last decade, is to use heat pipes or thermosyphons for transferring or spreading the dissipated heat. In this paper two-phase thermosyphon loops are discussed. Especially, the choice of fluid and its influence on the design and performance is treated. The discussion is supported by results from simulations concerning heat transfer and pressure drop. In general it is found that high-pressure fluids will give better performance and more compact designs as high-pressure results in higher boiling heat transfer coefficients and smaller necessary tube diameter.

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