Different Values Of Heat Flux Research Articles

Study on Convective Heat Transfer Characteristics of Supercritical Liquid Hydrogen in a U-Type Tube inside a Moderator

The flow and heat transfer characteristics of supercritical fluid in a U-tube have an important influence on the safe operation of a moderator, and the variation of gravity direction is suitable for special working conditions of the moderator. In this study, the three-dimensional turbulence flow and heat transfers of supercritical liquid hydrogen in a U-tube were investigated at an Re number ranging from 16,425 to 54,750 under constant heat flux (q = 80 kW/m2). The total length of the U-tube was 1725 mm, which had an entrance length L/D of 23, with the inner diameter and wall thickness of D × δ = 10 × 2 mm. The finite volume method was adopted, and the grid independence was verified by the grid convergence index (GCI). The calculation results of three turbulence models (SST k-w, RNG k-ε, Standard k-ε) were compared with the corresponding experimental data to obtain the turbulence model with the smallest error. The convective heat transfer characteristics with different values of heat flux (q = 30 kW/m2~100 kW/m2), mass flow (G = 3 g/s~10 g/s), and gravity (gx, gy, gz) were compared. Meanwhile, the heat transfer characteristics of supercritical liquid and conventional liquid hydrogen were compared. The results show that Nu increased from 5 g/s to 10 g/s by 56.6%, and mass flow rate had a greater impact on the variation of Nu; when gravity direction was consistent with the flow direction of liquid hydrogen (gx direction), the Nu number inside the channel was 4.21% and 5.56% higher than that in gy and gz direction, respectively. Supercritical liquid hydrogen has a stronger heat transfer ability than conventional liquid hydrogen, of which the Nu number is 16.7% higher. This study can provide useful guidance for the design of flow and heat transfer of supercritical liquid hydrogen in a U-tube and its application in moderators. Furthermore, it provides reference technical values for thermal safety and thermal management of the target station to ensure its safe and stable operation.

Experimental study on characteristics of gravity heat pipe with threaded evaporator

This work is for the first time an experimental study of thermal characteristics of an copper gravity heat pipe (thermosyrhon) with an outer diameter of 12 mm and a length of 230 mm with a threaded evaporator and freon R141b as working fluid under natural convection. The study demonstrates how a change in power input in the range from 5 W to 70 W (with a 5 W step) at natural convection affects such thermal characteristics of the gravity heat pipe (GPH) as temperature in evaporator section, temperature distribution along the length of the GPH, total thermal resistance, thermal resistance in evaporation and condensation sections, and heat transfer coefficients in evaporation and condensation sections. Also studied is the influence of tilt angle of the GPH changing from 0 to 90° (with a 15° step) on its thermal characteristics. The coefficient of convective heat exchange between the heat sink and air at different values of heat flux and tilt angle is determined experimentally. The obtained research results indicate that the GPH with a threaded evaporator can be effectively used in natural air cooling systems for electronic equipment with high heat flux values, for example, LED devices, power amplifiers of T/R modules, etc.

Effect of the solar roof chimney position on heat transfer in a room

This study reports a numerical-experimental analysis of heat transfer and airflow in a scaled room with a heated wall coupled with a double-channel vertical roof solar chimney. For the experimental study, a parametric study was performed in the thermal system, considering different values of heat flux supplied to a vertical wall of the scaled room and six different positions of the roof solar chimney. Experimental temperature profiles were obtained at six different depths and heights and experimental heat transfer coefficients were computed for both heated surfaces. The renormalization group k-ε turbulence model was evaluated against experimental data using computational fluid dynamics software. With the validated model, the effect of heat sources on temperature fields, flow patterns, and heat transfer convective coefficients is presented and discussed. The case with heat flux on the heated wall produces the biggest amount of air changes per hour, being the nearest position of the solar chimney to the air inlet the case with more air removal with 30.24 air changes per hour, corresponding to an output velocity of 0.13 m/s, compared to lower performance setting with 0.06 m/s muzzle velocity, likewise, this configuration presented the best thermal behavior for the configurations with and without the heated wall, with temperatures oscillating between 298 to 301 K. For this reason, it is important to consider this numerical-experimental (3D) study, to obtain the best use of natural ventilation and air changes per hour, offered by a double-channel roof solar chimney, being an efficient, economic, and ecological way of maintaining natural ventilation in buildings to live or work.

Computational Fluid Dynamics and Experimental Analysis of the Heat Transfer in a Room With a Roof Solar Chimney

Abstract This study reports a numerical-experimental analysis of heat transfer and airflow in a scaled room with a heated wall coupled with a double-channel vertical roof solar chimney. For the experimental part, a parametric study was performed in the thermal system, considering different values of heat flux supplied to a vertical wall of the scaled room (75 and 150 W/m2) and the absorber surface of the solar chimney (151 and 667 W/m2). Experimental temperature profiles were obtained at six different depths and heights, and experimental heat transfer coefficients were computed for both heated surfaces. The renormalization group k-ɛ turbulence model was evaluated against experimental data using computational fluid dynamics software. With the validated model, the effect of the heated wall and solar chimney on temperature fields, flow patterns, and heat transfer convective coefficients are presented and discussed. The cases with heat flux on the heated wall of the scaled room produce the biggest air changes per hour (ACH), being 30.1, 31.2, and 23.4 ACH for cases 1 to 3, respectively, while cases with no heated wall produce fewer ACH (11.72 and 12.28 for case 4 and 5). The comparison between cases with and without heat flux on one vertical wall but the same solar chimney heat flux shows that the ACH increases between 154% and 156%, respectively.

Thermal, efficiency and power output evaluation of pyramid, hexagonal and conical forms as solar panel

Through the present investigation, the thermal and power output of novel-shaped solar panels are evaluated. For the cooling of the mentioned forms, forced air flow was utilized. Three novel shapes, of Pyramid, Hexagonal, and Conical which had the equal lateral surface were considered. For the simulation, an open source CFD software was utilized. The lateral surfaces were put under identical amount of heat flux. Air as the coolant fluid was injected with constant inlet temperature from the trapdoors at the bottom of different shaped structures. Three different values of heat flux and air injection rate were evaluated for each shape. The outcomes presented that the conical shaped solar panel exhibits better thermal performance than other geometries. Furthermore, conical form finds the least temperature that was about 10.5 °C less than that of the pyramid-shaped panel. Furthermore, it was revealed that the corners of pyramid and hexagonal-shaped solar panels have higher temperature. Also, it was found that the efficiency of conical shaped panel was up to 8.4% more than that of pyramid-shaped panel.

Study of Flow and Heat Transfer in a Vertical Heated Duct as a Passive Solar Ventilation System.(Dept.M)

An experimental model for passive solar ventilating system is designed and constructed. This model 19 a rectangular vertical channel electrically heated from one of the two large sides and the other sides are exposed to ambient conditions. Due to uniform heating, a natural convective flow is produced. The induced air flow rate and average Nusselt number are calculated depending on the values of measured temperature of exit air and hot wall. The performance of the model at different values of heat flux as well; at different values of channel space ratio is examined. Channels of space ratio of 0.01, 0.02, 0.03, and 0.04 are tested at values of heat flux of 150,250 350, 50, 550, 65, and 750 wm2. Two proposed correlations for evaluation of Nusselt number and the mass flow rate are presented here.

Pool Boiling Heat Transfer from Aluminum Alloy Circular Surface Using Al2O3 and CuO Water Based Nano-fluids

The present work aims to study the effect of nano-particles volume fraction of nano-fluid on the heat transfer during pool boiling with different values of heat flux. The concentration ratios by volume in demineralized water are taken as 0.02 %, 0.20 %, 0.40 %, 0.60, and 0.80 % for Al2O3 nano-particles and 0.02 %, 0.06 %, and 0.20 % for CuO nano-particles. Heat transfer coefficients for pool boiling were established experimentally for different values of volume fraction and heat flux. The heating element is made from an aluminum alloy (AL 6061) with a circular smooth surface of 100 mm diameter and 10 mm thickness. The nano-particles porous layer that builds up during boiling is observed by a scanning electron microscope of the heated surface before and after the boiling. The results demonstrate that the heat transfer rate depends on the concentration ratios and heat flux. Using nano-particles decreases the pool boiling heat transfer in comparison with demineralized water. Due to the deposition of nano-particles on the heated surface, lower heat transfer is obtained for a lower bubble departure compared with demineralized water for the small wall superheat.

Influence of artificial disturbances on characteristics of the heated liquid film

The characteristics of a water film flowing down a vertical plate with a heater at high Reynolds numbers (Re=300 and 500) with and without artificial disturbances with the “most dangerous” wavelength have been studied experimentally. The field of film thickness at different values of heat flux has been measured using the fluorescent method. The temperature field on the liquid film surface was registered by the IR scanner.Artificial disturbances with the “most dangerous” wavelength decrease a distance between the rivulets and transverse deformation of liquid film. The temperature gradients and shear thermocapillary stresses on the film surface become smaller. Artificial disturbances with the “most dangerous” wavelength at high heat fluxes cause an increase in amplitude, spectral energy and integral energy of temperature pulsations per a time unit as well as expansion of the frequency range of temperature pulsations in-between the rivulets at the bottom of the heater. An increase in wave amplitude and velocity is observed in the interrivulet areas. An increase in wave amplitudes, velocities, and average integral pulsations of temperature per a time unit and frequency range expansion in the bottom part of the heater under the action of disturbances can enhance heat transfer and improve film resistance to breakdown.

Simplified model for polyurethane foaming in porous media

PurposeThe purpose of the study is to present a simplified model to replace the complicated foaming simulations for investigating the liquid polyurethane behavior just before solidification.Design/methodology/approachThis model is inspired from the traveling heater method of crystallization because of the low injection velocity. Besides, the heat generated during the reaction is considered as a heat source function in the energy equation.FindingsVarious distributions of the heat generation function inside the geometry have been studied to choose the most realistic one. Effect of parameters such as the soil material and porosity on the temperature distribution and flow field are examined for different values of heat flux on the boundaries. Results show an almost linear dependency of pressure drop to the velocity, a uniform velocity profile and an expected temperature distribution compared to literature, which approves the suggested model.Originality/valueA new model is presented in this study for foaming which replaces a heat generation function (exponential) in the source term of the energy equation instead of the heat produced at the exit boundary (the solid–liquid interface), and the traveling method is used instead of moving the geometry; besides, the growth ratio has been neglected; therefore, this model has been validated by a foaming simulation to confirm the suggested simplified idea.

On the optimization of the first wall of the DEMO water-cooled lithium lead outboard breeding blanket equatorial module

Within the framework of EUROfusion R&D activities a research campaign has been carried out at the University of Palermo in order to investigate the thermo-mechanical performances of the DEMO water-cooled lithium lead (WCLL) breeding blanket first wall (FW). The research campaign has been mainly focused on the optimization of the FW geometric configuration in order to maximize the heat flux it may safely withstand fulfilling all the thermal, hydraulic and mechanical requirements foreseen by safety codes. Attention has been focused on the FW flat concept endowed with square cooling channels and the potential influence of its four main geometrical parameters on its thermo-mechanical performances has been assessed performing a parametric analysis by means of a qualified commercial finite element method code. A set of 5929 different FW geometric configurations has been considered and the thermal performances of each one of them have been numerically assessed in case it undergoes 26 different values of heat flux on its plasma-facing surface. The resulting 154154 thermal analyses have allowed to select those cases fulfilling the adopted thermal-hydraulic requirements, whose thermo-mechanical performances have been numerically assessed under both normal operation and over-pressurization steady state loading scenarios to check whether they met the mechanical requirements prescribed by the pertaining SDC-IC safety rules. Four optimized FW configurations have been found to safely withstand a heat flux up to 2MW/m2 fulfilling all the rules prescribed by safety codes. Finally, the thermo-mechanical performances of these FW optimal configurations have been further investigated under both normal operation and over-pressurization steady state loading scenarios setting-up more realistic 3D FEM models. Results obtained, herewith presented and critically discussed, have allowed to select one promising FW geometrical configuration.

Thermal hydraulic simulation of absorber tubes in linear Fresnel reflector solar thermal system using RELAP

This paper presents thermal hydraulic modelling and simulation in the absorber tube of a Linear Fresnel Reflector (LFR) solar thermal system is carried out. The system is modelled using two-phase flow simulation software, RELAP5/MOD3.4. Although, RELAP5 is very commonly used in nuclear engineering design and simulation, it can be used for the simulation of solar thermal systems. Unlike other thermal systems like refrigerators and nuclear industries, there is significant heat loss from the surface of the absorber tube in a solar thermal system, which varies significantly with the temperature of the absorber tube wall. The recently developed temperature dependent heat loss has been incorporated in RELAP5 for variable net heat flux studies. The implementation of the temperature dependent heat losses has been verified by comparing the results obtained from RELAP with those obtained by Homogeneous Equilibrium Model. Parametric studies are carried out using verified RELAP model for different values of heat flux, mass flux, inlet subcooling and inlet pressure. The developed model can be considered as an effective tool for better and effective absorber LFR tube design under designed conditions.

Three‐Dimensional Numerical Investigation of Nanofluids Flow in Microtube with Different Values of Heat Flux

Forced convective laminar flow of different types of nanofluids such as Al2O3, CuO, SiO2, and ZnO, with different nanoparticle size 25, 45, 65, and 80 nm, and different volume fractions which ranged from 1% to 4% using ethylene glycol as base fluids were used. A three‐dimensional microtube (MT) with 0.05 cm diameter and 10 cm in length with different values of heat fluxes at the wall is numerically investigated. This investigation covers Reynolds number (Re) in the range of 80 to 160. The results have shown that SiO2‐EG nanofluid has the highest Nusselt number (Nu), followed by ZnO‐EG, CuO‐EG, Al2O3‐EG, and finally pure EG. The Nu for all cases increases with the volume fraction but it decreases with the rise in the diameter of nanoparticles. In all configurations, the Nu increases with Re. In addition, no effect of heat flux values on the Nu was found.

The effect of external disturbances on thermal entry length

The thermal entry length at high Reynolds numbers (Re = 300 and 500) in a water film flowing down a vertical plate with a heater in the absence and in the presence of artificial disturbances with the “most dangerous wavelength” has been studied experimentally. The field of film thickness at different values of heat flux has been measured using the fluorescent method. The data obtained using a high-speed infrared camera have shown that the thermal entry length increases under the influence of external disturbances in the inter-rivulet areas, which are formed between the crests of developed three-dimensional synchronous waves, and decreases for the rivulets.

PERFORMANCE OF WICKLESS HEAT PIPE WORKING WITH R134a

One of the important use of heat pipes is the thermal control of satellites. The performance characteristic of a wickless copper heat pipe was investigating experimentally. For that aspect, a test rig was built up for a heat pipe with 1.4 m length, 18 mm internal diameter and 2 mm thickness charged with R134 a as working fluid with a temperature range from -40 °C to 110 °C. The evaporator section was electrically heated with heat flux varying from 200 to 600 kW/m2. The condenser section was water cooled by using an overflow tank. Thermocouples were embedded on outside surface along the length of the heat pipe, and this surface was thermally insulated. The heat pipe transient responses as well as the axial distribution of wall temperature were plotted, for the three sections of the heat pipe, at different values of heat flux. The overall thermal conductance and the heat transfer coefficients at the evaporator and the condenser sections were plotted against the heat flux. These values were compared with the results published in literatures. The effect of some parameters on the heat pipe performance had been studied like the tilt angle and the adiabatic length of the pipe. The results show that the increase of the tilt angle or the adiabatic length of the pipe reduces its heat transport capacity. The correlation of Sakr and Ramadan [ 8 ] for the entrainment limit of heat pipes, had different values of n for R11, R12, R22 and R118 as working fluids. Applying the present results in this correlation, it had been found that n= 4.056 for wickless heat pipe with R134a.