Heat Flux Rate Research Articles

An experimental study of temperature and heat flux in a channel with an asymmetric thermal plume

A set of small-scale experiments were conducted to investigate the temperature and heat flux characteristics in a channel with an asymmetric thermal plume led by the restriction effect of sidewall. Rectangle heptane pools with different aspect ratios were burned to produce the thermal plume, and distance between the pool and sidewall was varied systematically. The classical McCaffery equation was modified to characterize the vertical flame temperature distribution under the channel ceiling. The ceiling temperature contours were analyzed by which the correlation between ceiling temperature, incident heat flux below, and ceiling flame length were investigated involving the sidewall effects. The dimensionless longitudinal heat flux distribution was correlated to the dimensionless distance to the pool using an exponential equation, while the decay rate of longitudinal heat flux was also investigated taking into account the pool aspect ratio. The heat flux at the fuel level for both symmetrical and asymmetrical flames can be predicted well by introducing a dimensionless heat release rate.

Analysis of conjugated processes of film evaporation and condensation in a tubular heat exchanger

ABSTRACTThis paper investigates coupled phase changes by forced convection from co‐current flows of liquid films and vapour‐air mixtures in a tubular heat exchanger. Evaporation occurs in the inner tube and condensation in the annulus region. Heat and mass transfers in the liquid films and the vapour‐gas mixtures are described by the conservation equations of momentum, energy, and diffusion. An appropriate change of coordinates is introduced in order to obtain a computational mesh with straight liquid‐mixture interfaces along the walls. The governing equations are discretized by an implicit finite differences method and solved using an efficient numerical methodology. The distributions of axial velocity, temperature, and vapour concentration in all regions of the tubes are determined from iterative procedures with a high convergence rate. The current analysis also provides the axial evolutions of film thicknesses, heat flux, condensation, and evaporation rates in the whole domain. In addition, the effects of the inlet parameters of Reynolds number, relative humidity, pressure, and liquid flow rate on the heat transfer characteristics, and the evaporation and condensation rates are also investigated.

Numerical simulation of heat transfer and determination of critical heat fluxes at nonsteady heat generation in falling wavy liquid films

In the current work, we present the mathematical model, which allows the calculation of the wave surface profile, velocity and temperature fields as well as the study of their evolution at a drastic change in heat loads with time. This model allows calculations of the wave characteristics and parameters of liquid film decay at different regime parameters.Using the presented model, we have simulated the wave formation process in falling film of liquid nitrogen and retrieved the resulting average wave characteristics for different inlet Reynolds numbers. We have also calculated the dependencies of boiling expectation time and total local evaporation time in falling wavy films of liquid nitrogen on heat flux density for different inlet Reynolds numbers.It was found out that all calculated dependencies are approximately similar in the area of high heat flux rates for all examined inlet Reynolds numbers indicating the weak influence of irrigation rate on boiling expectation time in this range of heat loads. Discrepancy of calculated dependencies was found in the area of low heat flux rates, what could be explained by significant influence of evaporation on heat transfer under such conditions. The dependencies of characteristic heat flux density of boiling suppression and total local evaporation suppression on the inlet Reynolds number were also presented.The regime map, which defines the different mechanisms of decay of falling wavy film of liquid nitrogen, was obtained by summing up the results of numerical simulation. The results of numerical simulations are in satisfactory agreement with experimental data.

Physiographical Study On the Extent of Effects Contributed by Soil Temperature and Humidity On Ground Heat Flux Rates

The ultimate objective of this research was to first assess the contribution of soil temperature and soil moisture on Ground Heat Flux (GHF). The specific objective was to determine GHF. To address these objectives, a field experiment was conducted in a vineyard located in Quebec, Canada during the spring of 2015. The results show that the change in soil temperature is of greater importance than soil humidity in the GHF variation range.

Experimental Investigation of Heat Transfer Characteristics and Cooling Performance of Mini‐ and Microheat Exchangers

AbstractThis paper presents an experimental investigation of forced convection heat transfer in two heat sinks for electronic system cooling and investigated the comparisons of the thermal behavior of the mini‐ and microchannel heat exchangers. The hydraulic dimension of one of the heat sinks is 2 mm while that of the other is . Deionized water was used as the working fluid for studies conducted in both the heat exchangers. The effect of heat flux and volumetric flow rate (in laminar flow regime) on temperature and heat transfer coefficient is studied. Irrespective of the average heat transfer coefficient and the total thermal resistance, advantages and limitations of each device are analyzed and discussed in the light of experimental results. Furthermore, the results obtained from the experiments were in good agreement with those obtained from the design theory analyses.

Regulation of Heat Exchange across the Hornbill Beak: Functional Similarities with Toucans?

Beaks are increasingly recognised as important contributors to avian thermoregulation. Several studies supporting Allen’s rule demonstrate how beak size is under strong selection related to latitude and/or air temperature (Ta). Moreover, active regulation of heat transfer from the beak has recently been demonstrated in a toucan (Ramphastos toco, Ramphastidae), with the large beak acting as an important contributor to heat dissipation. We hypothesised that hornbills (Bucerotidae) likewise use their large beaks for non-evaporative heat dissipation, and used thermal imaging to quantify heat exchange over a range of air temperatures in eighteen desert-living Southern Yellow-billed Hornbills (Tockus leucomelas). We found that hornbills dissipate heat via the beak at air temperatures between 30.7°C and 41.4°C. The difference between beak surface and environmental temperatures abruptly increased when air temperature was within ~10°C below body temperature, indicating active regulation of heat loss. Maximum observed heat loss via the beak was 19.9% of total non-evaporative heat loss across the body surface. Heat loss per unit surface area via the beak more than doubled at Ta > 30.7°C compared to Ta < 30.7°C and at its peak dissipated 25.1 W m-2. Maximum heat flux rate across the beak of toucans under comparable convective conditions was calculated to be as high as 61.4 W m-2. The threshold air temperature at which toucans vasodilated their beak was lower than that of the hornbills, and thus had a larger potential for heat loss at lower air temperatures. Respiratory cooling (panting) thresholds were also lower in toucans compared to hornbills. Both beak vasodilation and panting threshold temperatures are potentially explained by differences in acclimation to environmental conditions and in the efficiency of evaporative cooling under differing environmental conditions. We speculate that non-evaporative heat dissipation may be a particularly important mechanism for animals inhabiting humid regions, such as toucans, and less critical for animals residing in more arid conditions, such as Southern Yellow-billed Hornbills. Alternatively, differences in beak morphology and hardness enforced by different diets may affect the capacity of birds to use the beak for non-evaporative heat loss.

Analysis of Residual Stresses and Angular Distortion in Stiffened Cylindrical Shell Fillet Welds Using Finite Element Method

In this paper, a two-dimensional method is developed to simulate the fillet welds in a stiffened cylindrical shell, using finite element method. The stiffener material is aluminum 2519. The thermo-elasto-plastic analysis is used to analyze the thermo-mechanical behavior. Due to the high heat flux rate of the welding process, two uncouple thermal and mechanical analysis are carried out instead of performing a single couple thermo-mechanical simulation. In order to investigate the effects of the welding procedures, two different welding techniques are examined. The resulted residual stresses and distortions due to different welding procedures are obtained. Furthermore, this study employed the technique of element birth and death to simulate the weld filler variation with time in fillet welds. The obtained results are in good agreement with the published experimental and three-dimensional numerical simulation results. Therefore, the proposed 2D modeling technique can effectively give the corresponding results of 3D models. Furthermore, by inspection of the obtained residual hoop and transverse stresses and angular distortions, proper welding procedure is suggested.

Surface Deposition Characteristics of Supercritical Kerosene RP-3 Fuel within Treated and Untreated Stainless-Steel Tubes. Part 1: Short Thermal Duration

The thermal oxidation deposition characteristics of kerosene RP-3 have been experimentally studied in the vertical tube at supercritical pressure as a crucial concern for the cooled cooling air (CCA) development. Thermal stressing of the fuel was carried out in a heated tube with stainless steel 321 (SS321, 1Cr18Ni9Ti), pre-oxidized, and electrolytically passivated for 1 h. Under the constant pressure of 5 MPa, all of the experiments were conducted at the fixed inlet and outlet fuel temperatures of 400 and 723 K, respectively, under the same heat flux and flow mass rate. Deposition of the different segments was analyzed using a weighting method to observe the deposition profile of the test section. Moreover, the morphology and components of the surface deposition were examined along each tube, with different surface treatments, to investigate the surface thermal oxidative deposit mechanisms. In this work, it was found that the pre-oxidized and electrolytically passivated treatments could reduce the total ...

Transient Analysis of Microchannel Heat Sink during Single Phase and Flow Boiling Conditions

In the present work, an experimental investigation has been made to analyze the performance of microchannel heat sink under transient operating conditions. The transient analysis has been made by estimating the response time for different input heat flux and coolant mass flow rate. Analysis has been made for rectangular cross-section microchannels fabricated on a copper block of size 25.7 × 12 × 10 mm3. Twelve (12) numbers of microchannels are fabricated in the copper block. The width and depth of individual channels are 400 μm and 750 μm respectively. Performance analysis has been made for both single phase and flow boiling conditions of the coolant flow using deionized water as coolant. Experiments have been performed for coolant mass flux (G) range of 90 - 250 kg/m2s and input heat flux (q) range of 20 - 300 kW/m2 respectively. It has been observed that at constant input heat flux, response time decreases with the increase in coolant mass flux during single phase cooling. However this trend is not strongly followed during the two-phase or flow boiling cooling condition.

Monsoon Mixing Cycles in the Bay of Bengal: A Year-Long Subsurface Mixing Record

Based on the first year-long record of mixing collected in the eastern central Bay of Bengal, we quantify the role that subsurface turbulent heat fluxes play in upper-ocean cooling brought on by southwest (SW) and northeast (NE) monsoons. During the NE (dry, or winter) monsoon, atmospheric and subsurface turbulent heat fluxes each contribute about 50% of the net sea surface cooling. During the SW (wet, or summer) monsoon, the atmospheric heat flux varied widely due to “active” and “break” cycles of the monsoon intraseasonal oscillations, but had a net positive seasonal average. The subsurface turbulent heat flux during the SW monsoon led to surface cooling at rates more than three times greater than those measured during the NE monsoon. Since the seasonally averaged atmospheric heat flux was positive, subsurface mixing accounted for nearly all of the cooling during the SW monsoon. During the transition between the NE and SW monsoons, subsurface heat flux was near zero, and atmospheric heating rapidly warmed the sea surface. Following the SW monsoon, passage of Tropical Storm Hudhud led to O(1) m2 s–1 rates of turbulence diffusivity and strong subsurface heat flux, accounting for roughly half of the 1.4°C surface cooling that occurred over a 60-hour period.

Similarity Solutions for Laminar Boundary Layer Flow of Darcian Fluid over Horizontal Plate with a Convective Boundary Condition

This paper considers the problem of hydrodynamics and thermal boundary layers of Darcy flow over horizontal surface embedded in a porous medium. The solutions of such problems for the cases of uniform surface temperature and variable surface temperature have been studied and analysed in many papers. This paper, however, attempts to find similarity solutions for the Darcy flow problem with a convective boundary condition at the plate surface. It is found that the solution is possible when the heat transfer coefficient is proportional to x–2/3. The numerical solutions thus obtained are analyzed for a range of values of the parameter characterizing the hot fluid convection process. Analytical expressions are provided for local surface heat flux and total surface heat transfer rate while the flow variables are discussed graphically.

Description of heat transfer through a cylindrical liquid–vapor interface surface

Linear and nonlinear relations, which make it possible to determine the rate of heat flux in the mode of film boiling on a cylindrical heating surface, have been obtained by transforming the system of moment conservation equations derived from the Boltzmann kinetic equation based on the four-term approximation in the form of a two-sided Maxwellian. For this problem and the interaction potential of Maxwell molecules, the Boltzmann kinetic equation is solved by the moment method in elaboration of the approach of L. Lees and C.-Y. Liu to the description of thermal conductivity through a gas-filled cylindrical gap between two impermeable interfaces. The obtained analytical expressions for the heat flux can be used at any values of the Knudsen number determined from the cylindrical heater radius.

Aerothermal Postflight Analysis of the Sharp Edge Flight Experiment-II

The aerothermal instrumentation of the Sharp Edge Hypersonic Flight Experiment SHEFEX-II, which was launched from Andøya Rocket Range on top of a two-stage rocket configuration, provided very useful flight data. Complementary to the pressure data, heat flux rate and surface temperature were measured at selected locations on the payload along the complete ascent and descent trajectory. Measured heat flux rate and pressure fluctuations show excellent correlation with angle-of-attack variations. Calculated heat flux evolution using a hypersonic boundary-layer code along the trajectory is in reasonable agreement with measured heat flux data during flight. Boundary-layer transition was measured at a boundary-layer edge Reynolds number of approximately for both ascent and descent phases. The complementary analytical and numerical study for the flight rebuilding at selected flight points provided very useful data and allowed better understanding of the physical phenomena. Finally, a combined analytical–numerical approach was validated and applied for the computation of the heat flux rate along the flight trajectory. This verified method can be used for the design and analysis of future hypersonic flights of slender vehicles.

The effect of fuel area size on behavior of fires in a reduced-scale single-track railway tunnel

A set of experiments was carried out in a 1/9 reduced-scale single-track railway tunnel to investigate the effect of fuel area size on the temperature distribution and behavior of fires in a tunnel with natural ventilation. Methanol pool fires with four different fuel areas 0.6×0.3m2 (1 pan), 1.2×0.3m2 (2 pans), 2.4×0.3m2 (4 pans) and 3.6×0.3m2 (6 pans), were used in these experiments. Data were collected on temperatures, radiative heat flux and mass loss rates. The temperature distribution and smoke layer in the tunnel, along with overflow dimensions and radiant heat at the tunnel entrance were analyzed. The results show that as the fuel area enlarges, the fire gradually becomes ventilation-controlled and the ceiling temperature over the center of fire source declines. Burning at the central region of fire source is depressed due to lack of oxygen. This makes the temperature distribution along the tunnel ceiling change from a typical inverted V-shape to an M-shape. As observed in the experiments, a jet flame appeared at tunnel entrances and both the size and temperature of the flame increased with the enlargement of fuel area leading to a great threat to firefighters and evacuees in actual tunnel fires.

Design analysis of supercritical carbon dioxide recuperator

The segmental design method is employed to accurately capture the drastic variations of properties in the supercritical carbon dioxide (S-CO2) recuperator. The local heat capacity flow rates of both fluids have drastic changes in sub-heat exchangers even the mass flow rates of both fluids remain unchanged. When the heat duty is given, the local heat conductance, local temperature difference, local effectiveness and local entransy dissipation number have extremums, which appear in the vicinity of the heat capacity rate ratio is one. The heat transfer performance of recuperator improves at the expense of heat conductance. When the total heat conductance is fixed, there exist the maximum local heat flux rates, and the local effectiveness tends to be constant after the local heat capacity rate ratio reaches one, the local entransy dissipation number has the relatively coherent performance behavior in the sub-heat exchangers. The local heat capacity rate ratio has crucial influences on the heat transfer performance of recuperator, and the design parameters must be considered carefully in the design of S-CO2 recuperator.

Use of the Schlieren Method to the Convection Analysis in the Steel Charge of Mixed Porosity / Wykorzystanie Metody Schlierena Do Analizy Zjawiska Konwekcji W Przypadku Wsadu Stalowego O Porowatości Mieszanej

The paper presents experimental studies devoted to the convection phenomenon within the steel charge of mixed porosity. Such charges constitute bundles of hollow long elements such as pipes or rectangular sections which are heat treated. A significant portion of the gas phase in the volume of the charge makes that natural convection of the gas occurring within the individual elements may have an effect on the course of heating. To the tests the Schlieren method was used which is one of the optical visualization methods applied to the analysis of the flow phenomena in the transparent and non luminous media such as air or water. The tested samples have the form of porous charge beds made from pipes and rectangular profiles. During the experiments the samples were heating up for the constant heat flux rate. The direction of flux was vertical, from the bottom to the top.

Thermal properties of asphalt pavements under dry and wet conditions

Air voids play an important role in the temperature evolution of asphalt pavements exposed to the sun's radiation and the environment, but their effects have never been precisely quantified before. The objective of this article is to understand and quantify the effect of air voids on the temperature evolution of asphalt mixture under dry and wet conditions. To achieve this objective, dry and saturated asphalt slabs with different air voids contents have been exposed to infrared light and the surface and bottom temperature evolution, heat flux and evaporation rate have been measured. It was observed that under dry conditions air voids simply affect the specific heat capacity and thermal conductivity of asphalt mixture, while under wet conditions the energy used by water for evaporation reduces drastically the temperature of asphalt mixture. This has significance for Urban Heat Island mitigation amongst other implications.

Thermochemical storage performances of methane reforming with carbon dioxide in tubular and semi-cavity reactors heated by a solar dish system

Thermochemical storage performances of methane reforming with carbon dioxide in tubular and semi-cavity reactor heated by a solar dish system have been experimentally and numerically investigated. The methane conversion and thermochemical storage efficiency of methane reforming process in tubular reactor were experimentally studied for inlet flow rate 3–6L/min and direct normal irradiation (DNI) 677.8–714.3W/m2. According to the experimental system and results, Gaussian distribution model is derived for concentrated solar energy flux from solar dish, and a 3D finite volume method coupled with volumetric reaction kinetics and unilateral concentrated solar energy flux is established by experimental verification. The simulated methane conversion and energy storage efficiency have good agreements with the experimental data, and the temperature and species concentration distributions of the reactor are also successfully predicted. In the middle region of reactor, the concentrated solar energy flux and heat loss reach maxima, while the net heat flux and reaction kinetic rate reach maxima in the front region because of high temperature and high reactant fraction. As the catalyst bed length increases, the residence time and reverse reaction rate both rise, so there exists a proper catalyst bed length. When DNI rises, the methane reforming is promoted, while the heat loss remarkably increases, which results in the maximum thermochemical storage efficiency under proper DNI. Structural and operating parameters for the present tubular reactor can be further optimized, and the proper catalyst bed length is 300mm, while the proper DNI is 250–300W/m2 (focal energy flux of 244.3–293.2kW/m2). A semi-cavity reactor is designed to reduce the heat loss and enhance the energy storage performance. According to the experimental results under inlet flow rate 2–6L/min and DNI 452.4–598.5W/m2, the methane conversion of semi-cavity reactor can be increased to 74.8%, and the thermochemical storage efficiency and total energy efficiency can be respectively increased to 19.7% and 28.9%.

Heat balance evaluation of double-base solid propellant combustion using thermography and laser heating on a burning surface

A new method is proposed using laser heating and thermography for estimating the heat transfer balance for burning solid propellant: heat production rate on burning surface Q˙s, heat flux from burning surface to solid phase Λp, and heat flux rate from flame to burning surface Hf. Hf is determined from the correlation between burning rate and power density of laser heating on burning surface; Λp is evaluated using the burning surface temperature measured with thermography. Double-base propellant is tested in a combustion chamber having a laser-introducing window, and is irradiated with an 808-nm laser at laser power densities ranging from 0.3 to 0.8 W/mm2 with a back-pressure range from 0.02 to 0.60 MPa. The proposed method shows heat flux from burning surface to burning surface Hf of 1.7 W/mm2, heat production rate on burning surface Q˙s of 4.9 W/mm2, and heat flux from burning surface to solid phase Λp of 6.6 W/mm2 at a back pressure of 0.60 MPa. Laser power attenuation due to the flame is determined to evaluate the influence of laser power attenuation on the estimation of heat flux rate from flame to burning surface. The experiment shows that errors originating from laser power attenuation due to the flame are below 18%.

The dilemma of hyperbolic heat conduction and its settlement by incorporating spatially nonlocal effect at nanoscale

To model transiently thermal responses of numerous thermal shock issues at nano-scale, Fourier heat conduction law is commonly extended by introducing time rate of heat flux, and comes to hyperbolic heat conduction (HHC). However, solution to HHC under Dirichlet boundary condition depicts abnormal phenomena, e.g. heat conducts from the cold to the hot, and there are two temperatures at one location. In this paper, HHC model is further perfected with the aids of spatially nonlocal effect, and the exceeding temperature as well as the discontinuity at the wave front are avoided. The effect of nonlocal parameter on temperature response is discussed. From the analysis, the importance of size effect for nano-scale heat conduction is emphasized, indicating that spatial and temporal extensions should be simultaneously made to nano-scale heat conduction. Beyond that, it is found that heat flux boundary conditions should be directly given, instead of Neumann boundary condition, which does not make sense any longer for non-classical heat conductive models. And finally, it is observed that accurate solution to such problems may be obtained using Laplace transform method, especially for the time-dependent boundary conditions, e.g. heat flux boundary condition.