Quantity Of Heat Flux Research Articles

The effect of different variables and using of the internal adiabatic wall in the construction and performance of thermosiphon heat pipes: Experimental investigation

Heat pipes are a practical and powerful tool for recovering thermal energy and conserving energy sources. Thermosiphon is one of the most widely used devices that can transfer large amounts of heat at high rates between hot and cold sources without the use of external energy. The amount of vacuum in the pipe, the percentage of fluid filling, the type of operating fluid, the pipe’s length and the quantity of heat flux are the factors affecting the efficiency and effectiveness of the thermosiphon heat pipe. In this paper, the effects of different variables in the construction of heat pipes such as working fluid, pipe length, the use of mesh screen wick structure and the use of internal adiabatic wall on the thermosiphon heat pipes performance are investigated. The results show that using of an internal adiabatic wall eliminates and reduces limitations such as boiling, evaporator drying, thermosiphon flooding and vapor pressure and significantly improves the heat pipe’s performance. So that, the effective thermal conductivity (K) is increased up to 350% using the internal adiabatic wall. However, in some nanofluids, such as water/multi-walled carbon nanotubes (MWCNT), with increasing the nanofluid’s mass fraction, the startup speed in heat pipes with internal adiabatic wall is reduced by up to 20%.

Simulation of a Trombe wall with a number of semicircular fins placed on the absorber plate for heating a room in the presence of nano-PCM

This article numerically examines a Trombe wall for a mild winter climate. A number of semicircular fins are mounted on the Trombe wall. The inside of the fins is filled with phase change material (PCM). Graphene nanoparticles are also used in PCM. The wall is positioned at the south side of the building and is installed to heat a 4 × 4 room. Room and Trombe walls are simulated at 100, 300, and 500 W/m2 using ANSYS FLUENT software. The effect of factors such as Trombe wall height, distance between the glass wall and absorber plate, number of fins, presence or absence of PCM in fins, and flux quantity on average room temperature (TRA) is investigated. The findings reveal that the quantity of heat flux provided to the wall has a considerable influence on the room temperature (TRO), therefore increasing the applied heat flux raises the TRA. An increase in the air gap of the Trombe wall reduces the TRA. Adding fins by up to 11 enhances the TRO, but further increasing reduces the TRO. The use of PCM in the fins lowers the TRO during the day but stores the energy for the times with low solar flux and heats the room in the afternoon.

Direct numerical simulation of turbulent flow and heat transfer over a heated cube placed in a matrix of unheated cubes

Direct numerical simulation (DNS) of turbulent flow and heat transfer has been performed over a heated wall-mounted cube positioned in a non-heated cube matrix by applying periodic boundary conditions in both streamwise (X) and spanwise (Y) directions. Three different channel heights namely, 3.4H, 2.0H, 1.5H corresponding to blockage ratio (BR) of 0.0735, 0.125 and 0.167 with H as the size of the cube are employed. The Reynolds number defined for the present simulation is kept at 3,854 (based on the cube height and average streamwise velocity) while the Prandtl number is chosen to be 0.712. A second-order spatial and temporal discretization has been used to solve the Navier-Stokes and energy equations. The flow structures and the associated heat transfer have been compared and discussed at different BRs based on the results of instantaneous snapshots and statistical quantities of flow variables. The turbulent states for each BR has been compared using an anisotropic invariant map in the horseshoe vortex regime, top surface regime and in the wake regime. The total heat flux and turbulent heat flux quantities are compared near the cube’s surface to determine the contribution of both heat transfer by the thermal gradients, and due to the fluctuation induced heat flux. The overall Nusselt number is found to increase significantly with an increase in BR from 0.0735 to 0.125. However, at BR=0.167, no considerable augmentation in heat transfer has been observed as compared to BR=0.125. On the other hand, the friction factor increases monotonically but significantly with BR.

Technique, cell assembly, and measurement of T-dependent electrical resistivity of liquid Fe devoid of contamination at P, T conditions.

Since the cores of rocky planetary bodies are mainly Fe in composition, the understanding of the electrical resistivity and thermal conductivity of solid and molten Fe at pressure and temperature conditions is vital in placing a constraint on the quantity of heat flux from the cores of these planets. We develop an experimental technique and cell design to measure the temperature-dependent electrical resistivity of solid and molten Fe and other transition metals under high pressure. This addresses the problem of metal sample contamination encountered in designs that used W/Re, W, and Mo in direct contact with the sample. At first, we attempted to improve these pre-existing designs by testing the suitability of Hf and Zr metals to serve as a mechanical barrier between the electrodes and the sample. Unfortunately, our result shows that solid Hf and Zr dissolve in molten Fe and are not suitable for this purpose. Next, we adopt the same sample material, Fe, for electrodes and leads while the thermocouple leads are taken through the gasket and protected against frequent mechanical breakage using the shielding technique. The recovered Fe samples compressed at various pressure conditions and heated up to 200 K above the melting temperature show no trace of contamination. As anticipated, the resistivity increases and decreases with increasing temperature and pressure, respectively. Thus, to closely measure the electrical resistivity of molten Fe and other similar metals at extreme conditions, it is necessary to ensure liquid containment, eliminate biased voltage through the current reversal technique, and ensure the use of the same material for the electrode and sample while monitoring the sample temperature using a thermocouple placed close to but not in contact with the sample. Our developed technique provides the highly demanding technique for investigating the temperature-dependent electrical resistivity of Fe and other similar metals devoid of contamination at extreme conditions. This progress will accelerate studies which will provide a detailed understanding of the electrical and heat transport properties of Fe as it applies to the core of rocky planetary bodies.

Numerical investigations of impingement cooling performance on flat and non-flat targets with dimple/protrusion and triangular rib

Jet impingement is an efficient cooling method and has been widely adopted in electronic device and turbine blade. Well-designed structures including dimples and protrusions arranged in the flow path can alter flow fields in ways that provide a favorable trade between total pressure loss and increased heat transfer. In this paper, impingement cooling performances are numerically investigated for three different target shapes and various surface arrangements of dimple/protrusion and triangular rib. Within the Re range of 10,000–50,000, the detailed flow patterns, heat transfer and friction characteristics are obtained. Compared with flat channels, concave and V-shape targets produce more complex flow patterns. The adoption of dimple/protrusion improves the local and overall heat transfer. The Nu/Nu0 of 1.31 is achieved by sparse protrusion arrangement in concave and V-shape channels, while the f/f0 is only 0.96. The rib-dimple/protrusion arrangement improve the local Nu while significantly deteriorating the overall heat transfer. Comprehensively considering the quantity of heat flux Q, the Coefficient of Performance, Nu/Nu0 and f/f0, an excellent cooling performance has been achieved by protrusion arrangement in non-flat channels.

Analytical and Numerical Investigations of Physical Dimensions of Natural Convection Flow on a Vertical Heated Plate

The analytical and numerical results of a natural convection boundary layer flow on a vertical heated plate are presented. We specifically look at the effects of heat flux and plate temperatures on the development of a physical boundary layer along its heated surface. The plate temperature and heat flux examined are 9 ≤ T(°C) ≤ 100 and 50 ≤ qP ≤ 553 (W/m2) respectively. The results show that the variation in plate temperature has a significant effect on transition and the difference between the temperature of the air and the plate has even more effect on the transition particularly when TP ≤ 60°C. The transition on the constant heat-flux plate is less affected by air temperature and mostly depends on the quantity of heat flux. A Realizable k−e turbulent model with an enhanced wall function is employed in a numerical simulation, and important results, including the distribution of maximum velocity, and kinetic energy and its production along the heated plate, are examined for a selection of air temperatures. The results indicate that the production of kinetic energy reaches its peak at the transition stage, and both the velocity and turbulent kinetic energy increase sharply at the turbulent flow regime. Analytical and numerical results are compared with those of various relevant experimental studies and found to be in good agreement with them.

Force and thermal effects in vibration-assisted grinding

Because it reduces forces and temperatures, vibration-assisted grinding has the potential to improve the feasibility of dry grinding. This paper presents an experimental study of force and temperature effects in dry and wet grinding at vibration frequencies below ultrasonic. Based on a moving line heat source model, heat flux quantities were estimated from subsurface temperature measurements. Reductions in force of up to 30 % were observed for dry grinding with 2,360 Hz vibration assistance. For the same condition, heat flux into the workpiece reduced by 42 %. The paper presents evidence that vibration assistance has a beneficial effect on the convective heat transfer rate.

Mixed Convection in a Square Cavity Filled with Porous Medium with Heated Bottom Wall

Two-dimensional unsteady mixed convection in a porous cavity with heated bottom wall is numerically studied in the present paper. The forced flow conditions are imposed by providing a hydrostatic pressure head at the inlet port that is located at the bottom of one of the vertical side walls and an open vent at the top of the other vertical side wall. The Darcy model is adopted to model the fluid flow in the porous medium and the combination effects of hydrostatic pressure head and the heat flux quantity parameters are carefully investigated. These governing parameters are varied over wide ranges and their effect on the heat transfer characteristics is studied in detail. It is found that the time required to reach a desired temperature at the bottom wall decreases with heat flux and pressure head increase. The higher heat flux quantities leaves wider regions near the top wall at lower temperatures which is important in most engineering applications like drying.

A thermo-mechanical cohesive zone model for solder joint lifetime prediction

A Cohesive zone model (CZM) is developed for simulating the thermo-mechanical fatigue of solders in electronic chip packages subjected to active power cycling. Repeated on–off switching of the device is the source of thermal strains, resulting in the initiation and the propagation of a fatigue crack in the solder joint. The fatigue failure in the solder material may cause local intensification of the temperature gradient due to partial or total degradation of the thermal conductance in the cracked region. In this paper, the coupling between fatigue cracking and crack heat transfer is accounted for by using a thermo-mechanically coupled cohesive zone element. Thus, the quantity of heat flux transferred across the interface will be dependent on the degradation level of the interface thermal conductance. Typically, the cohesive zone conductance is taken as a function of the separation between the cohesive surfaces, meaning that the thermal conductance can be restored across the interface during crack closure. It may alternately be chosen as being dependent on the damage’s level of the cohesive zone element, describing in this way an irreversible thermal degradation in the solder joint. Simulations are carried out in order to analyze and predict the solder fatigue under active power cyclic loading. The numerical results reproduce qualitatively the desired behavior for the cracked interface and demonstrate how the temperature field and the heat transfer in the system are radically changed. Then, the evolution of the average damage level in the cohesive layer, reflecting the crack propagation level during power cycling, is used to estimate the number of cycles to failure at which the device becomes faulty.

Momentum and Heat Flux Measurements Using an Impact Target in Flowing Plasma

An impact target plate was used to determine the momentum and heat flux quantities from a helicon plasma source. The plasma source is the first stage of the Variable Specific Impulse Magnetoplasma Rocket, which uses radio frequency waves to create and energize a flowing plasma. The momentum and heat flux quantities are determined separately and agree favorably. They also agree with expected flux quantities calculated from the plasma parameters determined by other probes. The momentum and heat flux quantities were obtained at two axial locations in the exhaust or magnetic nozzle region of the device using helium and argon propellant. Only data from the plasma source are used, which shows that the plasma source does produce a flowing plasma with significant momentum. The flux data at the two axial locations verify the increase in momentum as the plasma flows through the diverging magnetic field in the magnetic nozzle region. Although the axial location for maximum momentum (or thrust) was not determined, forces of several millinewtons were measured for a combined neutral particle and plasma flow. The ionization fraction for these data was approximately 22 %. Neutral particles entrained in the flow provided a significant part of the flow momentum thereby contributing to the potential thrust. The technique of measuring the momentum flux and heat flux in a flowing plasma is described.

C132 固液間のエネルギー輸送に関する分子動力学的研究(OS-11 ナノスケール伝熱III)

The mechanism of thermal conduction between solid and liquid of single material was studied with the two-dimensional molecular dynamics method. The relation between the molecular movement and macroscopic thermal conduction was discussed under the assumption of stationary state. It was found that the size and the shape of a region defining a heat flux have little effect on the quantity of heat flux.

An experimental study of a two-phase closed loop thermosyphon with dual evaporator in parallel arrangement

A Two-phase Closed Loop Thermosyphon (TCLT) has been applied to many industrial fields as a waste heat recovery system, a telecommunication cooling system and other heat transport devices for the last 2 decades. It is common to design a TCLT to consist of one evaporator, one condenser and 2 separated lines of condensate liquid and vapor of working fluid. The present study was conducted in search of the new possibility of connecting multiple evaporators in parallel to the evaporator section. There has been little information about this so far. It is expected that a TCLT with multi-evaporator would provide a new solution to designing a compact telecommunication cooling system where lots of heat generating circuit boards are connected in parallel arrangement. In the present study a TCLT with two evaporators was prepared and given to a series of operational performance tests. The study was focused on investigating its operational problems and collecting information about its performances. Some important findings are as follows. The main source of operational instability was an occurrence of a hot dried patch on the heated surface when the rate of heat input was raised to a certain limit. For a single evaporator mode of the present TCLT model, the maximum allowable heat transport rate was about 650W. For dual evaporator mode of operation, this instability problem was found to be closely related not only to the quantity Of heat flux but also the difference in heat input rate to each evaporator. For all those problems, it was found that a TCLT with multi-evaporator could operate in a proper way when the thermal loads were evenly distributed to all the evaporator or the maximum power gap were contained under about ±5~7% of the average power input.

Pilot study using SPOT satellite imagery over Apalachicola national forest to determine appropriate spatial scale for area-wide aggregation of surface fluxes

High resolution radiances from SPOT satellite imagery converted to Normalized Difference Vegetation Indices (NDVI) over a 16×16 km2 mixed ground cover study-area in the Apalachicola National Forest in northwest Florida, along with in situ measurements from a Bowen ratio surface flux monitoring system and physical modeling techniques, are used to determine the length manifold beyond which degraded resolution satellite imagery fails to capture flux variability over the scene. The investigation is relevant to an understanding of how bias error is generated in methods designed to produce scale-invariant surface flux estimates from satellite measurements. Error estimates are based on assigning characteristic NDVI values to the four predominant types of ground cover found within the study-area. An open site near the center of the study-area, which satisfies the conditions for surface flux monitoring, is used for obtaining input data for a biosphere-atmosphere exchange model designed to calculate representative fluxes for the different ground covers. Continuous 6-minute meteorological and surface flux measurements were made at the monitoring site for a period of 22 days. These measurements are used in conjunction with surface layer theory to provide surface layer profile estimates of wind speed, temperature, and relative humidity at the tops of the forested sites. The measured and derived meteorological parameters, together with representative biophysical parameters, are used as input to the biosphere-atmosphere exchange model. By representing sensible and latent heat flux distributions due to the variable ground cover with characteristic NDVI values at 20-m resolution, baseline area-wide sensible and latent heat flux quantities are calculated. Error-growth curves as a function of spatial resolution for the fluxes are found by degrading the resolution of the SPOT radiances used to calculate NDVI, and rationing the associated area-wide fluxes to the baseline values. The point at which an error-growth curve becomes invariant represents the edge of a length manifold beyond which the satellite input no longer contains information on surface flux variability, even though NDVI variability continues at all scales up to that of the complete SPOT scene. The error-growth curves are non-linear, with all the error build-up taking place between 20 m and 1.6 km. Decreasing the spatial resolution of the NDVI information down to or below 1.6 km, introduces bias errors in the area-wide surface flux estimates of 10% for sensible heat and 8% for latent heat. The underlying assumptions and modeling produce uncertainty in estimating the manifold limits, however, the principal objective is to show that in using satellite data for scale-invariant surface flux retrieval, there is an optimal spatial resolution factor that can be objectively quantified.

A new diagram for evaluating the relationship between convective and evaporative heat transfer during the cooling of carcasses

During the cooling of a carcass, both its heat losses by convection and by evaporation are governed by the convective heat transfer coefficient αc. While both phenomena occur at the same temperature difference δ w between the surface of the carcass and the cold air, the quantity of heat flux of the former part becomes α cδ w, and the latter part becomes the product of α c multiplied by an equivalent temperature difference δ D which is a function of the vapour pressure differences between the wet surface and the cold air. The ratio δ D/δ w can be established from the diagram and the heat loss by convection and by evaporation can be calculated for any given value of α c. Consequently, the anticipated theoretical weight loss of the carcass can be calculated.