Free Convection Between Series of Vertical Parallel Plates With Embedded Line Heat Sources

1991 ◽  
Vol 113 (1) ◽  
pp. 108-115 ◽  
Author(s):  
S. H. Kim ◽  
N. K. Anand ◽  
L. S. Fletcher
Keyword(s):  
Boundary Condition ◽  
Nusselt Number ◽  
Surface Temperature ◽  
Mass Flow Rate ◽  
Mass Flow ◽  
Flow Rate ◽  
Parallel Plates ◽  
Cold Surface ◽  
Maximum Increase ◽  
Hot Surface

Laminar free convective heat transfer in channels formed between series of vertical parallel plates with an embedded line heat source was studied numerically. These channels resemble cooling passages in electronic equipment. The effect of a repeated boundary condition and wall conduction on mass flow rate (M), maximum surface temperature (θh,max and θc,max), and average surface Nusselt number (Nuh and Nuc) is discussed. Calculations were made for Gr*=10 to 106, K=0.1, 1, 10, and 100, and t/B=0.1 and 0.3. The effect of a repeated boundary condition decreases the maximum hot surface temperature and increases the maximum cold surface temperature. The effect of a repeated boundary condition with wall conduction increases the mass flow rate. The maximum increase in mass flow rate due to wall conduction is found to be 155 percent. The maximum decrease in average hot surface Nusselt number due to wall conduction (t/B and K) occurs at Gr*=106 and is 18 percent. Channels subjected to a repeated boundary condition approach that of a symmetrically heated channel subjected to uniform wall temperature conditions at K≥100.

Heat Transfer and Flow Friction Characteristics for Compact Cold Plates

2003 ◽  
Vol 125 (1) ◽  
pp. 104-113 ◽  
Author(s):  
Chang-Yuan Liu ◽  
Ying-Huei Hung
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Nusselt Number ◽  
Thermal Resistance ◽  
Mass Flow Rate ◽  
Mass Flow ◽  
Flow Rate ◽  
Average Nusselt Number ◽  
Cold Plate ◽  
Air Mass

Both experimental and theoretical investigations on the heat transfer and flow friction characteristics of compact cold plates have been performed. From the results, the local and average temperature rises on the cold plate surface increase with increasing chip heat flux or decreasing air mass flow rate. Besides, the effect of chip heat flux on the thermal resistance of cold plate is insignificant; while the thermal resistance of cold plate decreases with increasing air mass flow rate. Three empirical correlations of thermal resistance in terms of air mass flow rate with a power of −0.228 are presented. As for average Nusselt number, the effect of chip heat flux on the average Nusselt number is insignificant; while the average Nusselt number of the cold plate increases with increasing Reynolds number. An empirical relationship between Nu¯cp and Re can be correlated. In the flow frictional aspect, the overall pressure drop of the cold plate increases with increasing air mass flow rate; while it is insignificantly affected by chip heat flux. An empirical correlation of the overall pressure drop in terms of air mass flow rate with a power of 1.265 is presented. Finally, both heat transfer performance factor “j” and pumping power factor “f” decrease with increasing Reynolds number in a power of 0.805; while they are independent of chip heat flux. The Colburn analogy can be adequately employed in the study.

scholarly journals Optimal design to control rotor shaft vibrations and thermal management on a supercritical CO2 microturbine

2021 ◽  
Vol 22 ◽  
pp. 22
Author(s):  
Jun Li ◽  
Hal Gurgenci ◽  
Jishun Li ◽  
Lun Li ◽  
Zhiqiang Guan ◽  
...  
Keyword(s):  
Boundary Condition ◽  
Mass Flow Rate ◽  
Mass Flow ◽  
Flow Rate ◽  
Jet Impingement ◽  
Brayton Cycle ◽  
Cooling Device ◽  
Resonance Amplitude ◽  
Chaotic Mapping ◽  
The Given

Supercritical carbon dioxide (SCO2) Brayton cycle microturbine can be used for the next generation of solar power. In order to comprehensively optimize the supporting system and cooling device parameters of Brayton cycle shafting, the concept of chaos interval is introduced by chaotic mapping, and the CIMPSO algorithm is proposed to optimize the multi-objective rotor system model with nonlinear variables.The results show that the resonance amplitude of the optimized model is effectively attenuated, and the critical speed point is far away from the working speed, which shows the robustness of the optimization algorithm. Finally, based on arbitrary several sets of optimization solutions and empirical parameters, the finite element model of shafting is established for simulation, and the results show that the optimized solution has certain guiding significance for the design of the rotor system.The cooling device is designed and simulated by CFD method based on the optimal solution set. Both the inlet boundary conditions of given pressure (1 MPα) and given mass flow rate (0.1 kg/s) numerical calculations were carried out to characterize the cooling performance, for different jet impingement configurations (Hr/din = 0.0125 ∼ 5).Several sets of analyses show the strong effects of the jet-to-target spacing (Hr/din) on the rotor thermal performance at a given diameter (din) of the nozzle. Average temperature (Tc) at the free end of the rotor show that, as jet-to-target distance decreases (0.0125 ≤ Hr/din ≤ 0.33), the heat dissipation efficiency of the cooling device with the given pressure boundary condition tends to decrease, while the conclusion is opposite when the inlet boundary condition is set to the given mass flow rate. And there is an interval for the optimal combination (Hr/din) to promote the cooling efficiency.

scholarly journals Non-equilibrium dynamics of dense gas under tight confinement

2016 ◽  
Vol 794 ◽  
pp. 252-266 ◽  
Author(s):  
Lei Wu ◽  
Haihu Liu ◽  
Jason M. Reese ◽  
Yonghao Zhang
Keyword(s):  
Boltzmann Equation ◽  
Flow Regime ◽  
Mass Flow Rate ◽  
Knudsen Number ◽  
Mass Flow ◽  
Flow Rate ◽  
Transitional Flow ◽  
Molecular Flow ◽  
Parallel Plates ◽  
The Boltzmann Equation

The force-driven Poiseuille flow of dense gases between two parallel plates is investigated through the numerical solution of the generalized Enskog equation for two-dimensional hard discs. We focus on the competing effects of the mean free path ${\it\lambda}$, the channel width $L$ and the disc diameter ${\it\sigma}$. For elastic collisions between hard discs, the normalized mass flow rate in the hydrodynamic limit increases with $L/{\it\sigma}$ for a fixed Knudsen number (defined as $Kn={\it\lambda}/L$), but is always smaller than that predicted by the Boltzmann equation. Also, for a fixed $L/{\it\sigma}$, the mass flow rate in the hydrodynamic flow regime is not a monotonically decreasing function of $Kn$ but has a maximum when the solid fraction is approximately 0.3. Under ultra-tight confinement, the famous Knudsen minimum disappears, and the mass flow rate increases with $Kn$, and is larger than that predicted by the Boltzmann equation in the free-molecular flow regime; for a fixed $Kn$, the smaller $L/{\it\sigma}$ is, the larger the mass flow rate. In the transitional flow regime, however, the variation of the mass flow rate with $L/{\it\sigma}$ is not monotonic for a fixed $Kn$: the minimum mass flow rate occurs at $L/{\it\sigma}\approx 2{-}3$. For inelastic collisions, the energy dissipation between the hard discs always enhances the mass flow rate. Anomalous slip velocity is also found, which decreases with increasing Knudsen number. The mechanism for these exotic behaviours is analysed.

scholarly journals Modeling the Performance of Bi-Textured Micropillar Array as a Wicked Evaporator

2016 ◽  
Author(s):  
Hassan Azarkish ◽  
Amin Behzadmehr ◽  
Luc G. Frechette
Keyword(s):  
Surface Temperature ◽  
Mass Flow Rate ◽  
Mass Flow ◽  
Flow Rate ◽  
Pillar Array ◽  
Thin Film Evaporation ◽  
Pillar Arrays ◽  
The Stability ◽  
Micro Pillars ◽  
Evaporation Mechanism

In the present work, the performance of bi-textured micro pillar arrays has been modeled as a wicked evaporator to provide steam flow via the thin film evaporation mechanism. Bi-textured micro pillar evaporator consists of an array with rough hydrophilic pillar bases and smooth hydrophobic tips. Water wicks between the rough hydrophilic sections of the micro pillar array to cover the surface, and vaporizes from the thin films that are formed in the vicinity of the pillar walls. The stability of the phase change mechanism is increased due to the change in direction of the capillary forces at the rough-smooth interface of micro pillars. The experimental results show that the pure evaporation mechanism occurs for a surface temperature above saturation on the bi-textured micro pillar array. The numerical analysis shows that there are optimal micro pillar dimensions for each surface temperature. The evaporation mass flow rate at the optimum dimensions is higher than the pool boiling mass flow rate on a bare surface at the same surface temperature. However, the wicked evaporator performance decreases for larger evaporator sizes.

Mass Flow Rate Measurement Through Rectangular Microchannels for Large Knudsen Number Range

2011 ◽  
Author(s):  
M. Hadj Nacer ◽  
Pierre Perrier ◽  
Irina Graur
Keyword(s):  
Boundary Condition ◽  
Mass Flow Rate ◽  
Knudsen Number ◽  
Mass Flow ◽  
Flow Rate ◽  
Stokes Equations ◽  
Rectangular Cross Section ◽  
Velocity Slip ◽  
Bgk Model ◽  
Number Range

The mass flow rate through microchannels with rectangular cross section is measured for the wide Knudsen number range (0.0025–26.2) in isothermal steady conditions. The experimental technique called ‘Constant Volume Method’ is used for the measurements. This method consists of measuring the small pressure variations in the tanks upstream and downstream of the microchannel. The measurements of the mass flow rate are carried out for three gases (Helium, Nitrogen and Argon). The microchannel internal surfaces are covered with a thin layer of gold with mean roughness Ra = 0.87nm (RMS). The continuum approach (Navier-Stokes equations) with first order velocity slip boundary condition was used in the slip regime (Knudsen number varies from 0.0025 to 0.1) to obtain the experimental velocity slip and accommodation coefficients associated to the Maxwell kinetic boundary condition. In the transitional and near free molecular regimes the linearized kinetic BGK model was used to calculate numerically the mass flow rate. From the comparison of the numerical and measured values of the mass flow rate the accommodation coefficient was also deduced.

scholarly journals Numerical Study of a Thermosyphon Cooling System: film condensation

2018 ◽  
Vol 42 ◽  
pp. 01005
Author(s):  
Filian Arbiyani
Keyword(s):  
Nusselt Number ◽  
Standard Deviation ◽  
Water Temperature ◽  
Mass Flow Rate ◽  
Mass Flow ◽  
Flow Rate ◽  
Reynold Number ◽  
Film Condensation ◽  
Constant Mass ◽  
Cooling Systems

Studies of condensation in several cooling systems have been conducted. However, the mode of condensation in two-phase cooling systems to achieve a high rate of condensation in compact devices has not been explored. Condensation phenomena, indeed, is a key parameter in designing a thermosyphon water cooled condenser system. The analysis of this condensation phenomena has been done numerically by implementing the governing equations and boundary conditions in commercial MATLAB software. Steady-state laminar film condensation on the radial system is assumed as a condensation phenomenon between vapor and the outer surface of coolant coil. There is a good agreement between experimental and simulation results. Furthermore, for 0.3 LPM 10 °C, it is found the standard deviation of 0.3 %. This small standard deviation indicates the good accuracy of the simulation. At a constant mass flow rate of water, the higher inlet water temperature will result in a higher Nusselt number of water. Furthermore, at the same Nusselt number of water, the lower inlet water temperature obtained a higher film condensation rate. Nusselt number of film condensation increases as the Nusselt number of water decreases at the various constant of mass flow rate of water. Additionally, the lower inlet water temperature will result in a lower Nusselt number of water. The value of Reynold number film condensation increases as Reynold numbers and Nusselt number of water increase. At various constant mass flow rates of the water, at the same Nusselt number of water, the Reynold number of film condensation increases with lower inlet water temperature. The lower inlet water temperature increases the value of Reynold number of film condensation leading to more wavy and turbulent flow. The present study provides guidelines for thermal management engineers to design and fabricate compact cooling systems.

scholarly journals Active Insulation Technique Applied to the Experimental Analysis of a Thermodynamic Control System for Cryogenic Propellant Storage

2016 ◽  
Vol 8 (2) ◽  
Author(s):  
Samuel Mer ◽  
Jean-Paul Thibault ◽  
Christophe Corre
Keyword(s):  
Boundary Condition ◽  
Mass Flow Rate ◽  
Mass Flow ◽  
Flow Rate ◽  
Heat Load ◽  
Thermal Power ◽  
Pressure Rise ◽  
Electrical Heating ◽  
Power Balance ◽  
Flux Boundary Condition

A technological barrier for long-duration space missions using cryogenic propulsion is the control of the propellant tank self-pressurization (SP). Since the cryogenic propellant submitted to undesired heat load tends to vaporize, the resulting pressure rise must be controlled to prevent storage failure. The thermodynamic vent system (TVS) is one of the possible control strategies. A TVS system has been investigated using on-ground experiments with simulant fluid. Previous experiments performed in the literature have reported difficulties to manage the thermal boundary condition at the tank wall; spurious thermal effects induced by the tank environment spoiled the tank power balance accuracy. This paper proposes to improve the experimental tank power balance, thanks to the combined use of an active insulation technique, a double envelope thermalized by a water loop which yields a net zero heat flux boundary condition and an electrical heating coil delivering a thermal power Pc∈[0−360] W, which accurately sets the tank thermal input. The simulant fluid is the NOVEC1230 fluoroketone, allowing experiments at room temperature T ∈ [40–60] °C. Various SP and TVS experiments are performed with this new and improved apparatus. The proposed active tank insulation technique yields quasi-adiabatic wall condition for all experiments. For TVS control at a given injection temperature, the final equilibrium state depends on heat load and the injection mass flow rate. The cooling dynamics is determined by the tank filling and the injection mass flow rate but does not depend on the heat load Pc.

Experimental Studies on Nanofluid-Based Rectangular Natural Circulation Loop

2019 ◽  
Vol 11 (4) ◽  
Author(s):  
Ramesh Babu Bejjam ◽  
K. Kiran Kumar ◽  
Karthik Balasubramanian
Keyword(s):  
Nusselt Number ◽  
Steady State ◽  
Rayleigh Number ◽  
Mass Flow Rate ◽  
Mass Flow ◽  
Flow Rate ◽  
Thermal Performance ◽  
Natural Circulation ◽  
Working Fluids ◽  
Natural Circulation Loop

The main objective of the present study is to carry out experimental investigation on thermal performance of the nanofluid-based rectangular natural circulation loop (NCL). For this study, an experimental test rig is fabricated with heater as heat source, and tube in tube heat exchanger as heat sink. For the experimentation, three different nanofluids are used as working fluids. The nanometer-sized particles of silicon dioxide (SiO2), copper oxide (CuO), and alumina (Al2O3) are dispersed in distilled water to produce the nanofluids at different volume concentrations ranging from 0.5% to 1.5%. Experiments are carried out at different power inputs and different cold fluid inlet temperatures. The results indicate that NCL operating with nanofluid reaches steady-state condition quickly, when compared to water due to its increased thermal conductivity. The steady-state reaching time is reduced by 12–27% by using different nanofluids as working fluids in the loop when compared to water. The thermal performance parameters like mass flow rate, Rayleigh number, and average Nusselt number of the nanofluid-based NCL are improved by 10.95%, 16.64%, and 8.10%, respectively, when compared with water-based NCL. At a given power input, CuO–water nanofluid possess higher mass flow rate, Rayleigh number and Nusselt number than SiO2–water and Al2O3–water nanofluids due to better thermo-rheological properties.

Numerical Research on Performance of AP1000 Passive Containment Cooling System Without Falling Film

2013 ◽  
Author(s):  
Hui Wang ◽  
Qiang Guo ◽  
Qiaoyan Chen
Keyword(s):  
Natural Convection ◽  
Thermal Radiation ◽  
Surface Temperature ◽  
Mass Flow Rate ◽  
Mass Flow ◽  
Flow Rate ◽  
Environmental Temperature ◽  
Cooling System ◽  
Heat Removal ◽  
Falling Film

Passive Containment Cooling System (PCS) is one of the most important passive safety features in AP1000 nuclear power plant. Numerical Simulation of PCS without falling film was conducted using CFD method in this paper to investigate heat transfer performance of PCS. The results indicate that: at the time of 72 hours after break accident, there is no water in passive containment cooling water storage tank (PCCWST), PCS cannot completely remove reactor core decay heat to environment just with the effect of natural convection and thermal radiation; mass flow rate of natural convection and PCS heat removal power increase with the increase of the outside surface temperature of steel containment, but the increasing amplitude of the former is smaller than the latter significantly, and the fraction of thermal radiation in the PCS heat removal power is the smallest when the outside surface temperature of steel containment is 80 °C; mass flow rate of natural convection and PCS heat removal power decrease with the increase of environmental temperature, while the fraction of thermal radiation in the whole heat removal power becomes bigger with the increase of environmental temperature.

The standard unit mass flow rate of gas-liquid mixtures

2020 ◽  
pp. 13-16
Author(s):  
V.N. Petrov ◽  
◽  
V.F. Sopin ◽  
L.A. Akhmetzyanova ◽  
Ya.S. Petrova ◽  
...  
Keyword(s):  
Mass Flow Rate ◽  
Mass Flow ◽  
Flow Rate ◽  
Liquid Mixtures ◽  
Unit Mass ◽  
Standard Unit
Sign in / Sign up

Export Citation Format

Share Document