Cumulative Heat Flux Research Articles

Summer thermal and energy performances assessment of a modular hydronic thermal barrier wall for ultra-low energy buildings - A field experimental study

Hydronic thermal barriers with ultra-low carbon emission attributes are progressively gaining prominence in the competition of thermal insulation solutions for opaque envelopes of low-energy buildings due to their excellent thermal performance and low-grade energy utilization potential. This paper introduces a novel approach involving a modular hydronic thermal barrier wall (MHTB) that incorporates preset pipe cavities and replaceable filling material. This proposition seeks to address the challenges confronted by conventional hydronic thermal barrier walls (CHTBs), specifically in terms of internal heat transfer enhancement and rapid assembly/dismantling. On-site thermal performance tests and energy-saving evaluations were conducted in summer conditions based on three scaled-down rooms with MHTB, CHTB and a conventional external insulation wall as the south walls. The results showed that the MHTB could significantly reduce heat passing through the south wall in summer, resulting in a decreased cooling load attributed to the wall and subsequently lowering indoor temperatures. At a charging temperature of 17.0 °C, the MHTB exhibited a reduction of 11.3 °C in interior surface temperature compared to the conventional external insulation wall, accompanied by a corresponding increase of 2.76 kWh·m−2 in cumulative heat flux. In addition, the MHTB could effectively mitigate inhomogeneous heat transfer within the wall, enhancing the overall heat transfer effect. Across the four investigated conditions, the MHTB with a steel grit-sand mixture as the filling material demonstrated an approximate 4.7% increase in cumulative heat flux and a 12.2% enhancement in cumulative cooling quantity compared to the CHTB. Overall, the findings substantiated the efficacy of the MHTB system and offered a crucial point of reference for advancing research in optimizing the performance and modular construction of hydronic thermal barrier walls.

Numerical simulation of erosion of misaligned W/Cu monoblocks caused by ITER-like type-Iedge localized modes

The thermal performance of the divertor W/Cu monoblock tiles under the ITER-like transient events has been one of the main concerns for ITER plasma facing components. Owing to the assembly tolerances during installation, the leading edge caused by misalignment between toroidal neighboring tiles will receive the extremely high cumulative heat flux and be damaged. In this work, we develop a two-dimensional heat conductivity model, including evaporation, radiation, melting process, and coupling cooling water condition, to investigate the thermal erosion of two shapes of tiles of W/Cu monoblock (unshaped and beveled tiles) with misalignment in a range from 0 to 0.3 mm, within the allowable maximum misalignment for ITER. To reflect the geometrical effects of castellated divertor tiles on the properties of its adjacent plasma, the energy flux density distribution arriving at the castellated divertor tile surface is evaluated first by using a two-dimension-in-space and three-dimension-in-velocity particle-in-cell plus Monte Carlo collisions code, and the obtained energy flux distribution is then used as input for the heat conduction model. The simulation results show that the maximum temperature of the unshaped tile with no misalignment is lower than that of the beveled tile under the steady-state inter-ELM heat flux, which increases more quickly than that of the beveled tile with misalignment increasing and will be larger than that of the beveled tile when misalignment is not less than 0.105 mm. Two shapes of the divertor tiles would melt and vaporize under typical heat flux density of a transient event of type-I edge localized modes (ELMs) for ITER, deposition energy of 1 MJ·m<sup>–2</sup> in a duration of 600 μs. The highest temperature, the maximum melting thickness, the maximum vaporization thickness of the unshaped tile with no misalignment are higher than those of the beveled tile except the melting volume ratio. The thermal erosion of the unshaped tile increases more remarkably than that of the beveled tile with misalignment increasing, and the melting volume ratio of the unshaped tile will exceed that of the beveled tile at a misalignment of 0.17 mm. In comparison with the unshaped tile, the beveled tile is more resistant to such a high heat flux of an ELM and misalignment.

Thermal behavior prediction and adaptation analysis of a reelwell drilling method for closed-loop geothermal system

The ultra-high temperature characteristic of the deep closed-loop geothermal system poses a great challenge for drilling. This paper proposes a reelwell drilling method for temperature-controlled drilling in ultra-high temperature geothermal reservoirs. Combining the flow characteristics and heat transfer mechanisms in each flow channel under three circulating modes of reelwell drilling, a set of integrated transient heat transfer models is developed for distinct thermal-associated regions. The model simultaneously couples the effect of the variable temperature-mass flow resulting from the fluid transition in different flow channels. The finite difference method is used to solve the model, and the field measurement data is employed to validate the model. The heat transfer rate and wellbore temperature distribution in reelwell drilling are investigated, and the optimum circulating mode suitable for temperature-controlled drilling is optimized. The results indicate that the cumulative bottomhole heat flux is always negative in reelwell drilling, revealing that the bottomhole temperature decreases gradually. Additionally, the absolute values of the cumulative heat flux under different circulating modes are consistent with circulating mode B > conventional drilling > circulating mode C > circulating mode A. Thus, the circulating mode B can effectively reduce the bottomhole temperature during drilling directional well. Moreover, when a suitable dual-channel valve position is selected in the middle and lower part of the wellbore and the auxiliary fluid inlet temperature is controlled to be less than 20 °C, the circulating mode B has the best temperature-controlled effect. Therefore, the circulating mode B of reelwell drilling method can provide a new option for temperature-controlled drilling for the deep closed-loop geothermal system.

The influence of variable emissivity on lava flow propagation modeling

Modeling lava flow propagation is important to determine potential hazards to local populations. Thermo-rheological models such as PyFLOWGO track downflow cooling and rheological responses for open-channel, cooling-limited flows. The dominant radiative cooling component is governed partly by the lava emissivity, which is a material property that governs the radiative efficiency. Emissivity is commonly treated as a constant in cooling models, but is shown here to vary with temperature. To establish the effect of temperature on emissivity, high spatiotemporal, multispectral thermal infrared data were acquired of a small flow emplaced from a tumulus. An inverse correlation between temperature and emissivity was found, which was then integrated into the PyFLOWGO model. Incorporating a temperature-dependent emissivity term results in a ∼5% increase in flow length and < 75% lower total cumulative heat flux for the small flow. To evaluate the scalability of this relationship, we applied the modified PyFLOWGO model to simulations of the 2018 Lower East Rift Zone fissure 8 flow, emplaced between May 27 and June 3. Our model improves the emplacement match because of the ~ 30% lower heat flux resulting in a ∼7% longer flow compared to modeling using a constant emissivity (0.95). This 5–7% increase in length prior to ocean entry, realized by an accurate temperature-dependent emissivity term, is critical for developing the most accurate model of future flow hazard assessments, particularly if population centers lie in the flow’s path.

The thermal performance of a green roof on a highly insulated building in a sub-arctic climate

Green roofs are complex systems, with a vegetation layer covering the outermost surface of the building shell. An effective design may confer environmental and energy benefits. Most field studies evaluating green roof performance have been conducted in warmer climates with few studies of full-scale green roofs in cold regions. No study has so far evaluated the energy performance of a green roof in a sub-arctic climate. This study demonstrates the heat flow and thermal effect of an extensive green roof versus a black bare roof area on a highly insulated building in the sub-arctic town of Kiruna, Sweden, for the period from November 2016 to February 2018. Measured temperature and heat flux values were consistently higher and more variable for the black roof than the green roof, except during the snow-covered winter months when the responses were similar. The cumulative heat flux showed that the net heat loss was greater through the black than the green roof, but the values remained low. Overall, the study confirms that the energy benefit of a green roof on a highly insulated building in a subarctic climate is low.

Experimental and numerical study on the thermal performance of ventilated roof composed with multiple phase change material (VR-MPCM)

In the hot summer, roof is the prime location absorbed the most solar radiation for both residential and commercial buildings. Hence, enhancing the thermal performance of roofs is essential for reducing the heat gain thus building load. In this study, a novel ventilated roof composed with multiple phase change material (VR-MPCM) was proposed and its thermal performance was investigated experimentally and numerically. A contrast experiment was conducted in two full-scale chambers. The research results indicated that VR-MPCM can reduce the indoor peak temperature by 16.9–18.8%, and push back the peak temperature occurrence time by 30–50 min compared with the conventional ventilated roof (CVR). In addition, the cumulative heat flux during 4320 min demonstrated that the energy conservation rate of VR-MPCM can go up to 97.1% compared with the CVR. A verified three-dimensional transient heat transfer model was introduced for further research. Through the simulation research, intermittent opening of vents was found to be the best operating strategy for reducing heat gain. Regional comparison indicated that VR-MPCM in severe cold and cold climate zones had obvious energy saving potential. In conclusion, VR-MPCM has a potential of energy saving by reducing the diurnal heat gain and utilizing the nightly free cooling.

Temperature Measurements on a Solar and Low Enthalpy Geothermal Open-Air Asphalt Surface Platform in a Cold Climate Region

Solar heat, already captured by vast asphalt fields in urban areas, is potentially a huge energy resource. The vertical soil temperature profile, i.e., low enthalpy geothermal energy, reveals how efficiently the irradiation is absorbed or radiated back to the atmosphere. Measured solar irradiation, heat flux on the asphalt surface and temperature distribution over a range of depths describe the thermal energy from an asphalt surface down to 10 m depth. In this study, those variables were studied by long-term measurements in an open-air platform in Finland. To compensate the nighttime heat loss, the accumulated heat on the surface should be harvested during the sunny daytime periods. A cumulative heat flux over one year from asphalt to the ground was 70% of the cumulative solar irradiance measured during the same period. However, due to the nighttime heat losses, the net heat flux during 5 day period was only 18% of the irradiance in spring, and was negative during autumn, when the soil was cooling. These preliminary results indicate that certain adaptive heat transfer and storage mechanisms are needed to minimize the loss and turn the asphalt layer into an efficient solar heat collector connected with a seasonal storage system.

Integrated optical-thermal-mechanical model for investigations into high temperature sodium receiver operation

An integrated optical-thermal-mechanical model is used to simulate the interaction between a heliostat field and sodium-cooled receiver through diurnal CSP plant operation. The cumulative heat flux on the receiver is controlled using a heliostat aiming strategy. A semi-empirical heat transfer model returns the receiver thermal profile based on the incident heat flux map, with aiming strategy solutions regulated by allowable flux density (AFD) data. A number of tube materials and inlet-outlet temperature combinations are investigated, with receiver thermal power output indicating suitable constructions for the delivery of elevated temperatures required for advanced power cycles. The selection of tube material is critical in maximising the heat absorption capabilities. The excellent creep-fatigue strength of Inconel 617 and Haynes 230 at high temperature yields a potential improvement in daily power output of up to 18% over more conventional heat exchanger materials. For these more traditional alloys, the aiming strategy requires a greater spillage allowance in order to generate a heat flux profile that satisfies lower mechanical reliability limits. The Ni-based superalloys alloys permit operation to temperatures far beyond conventional limits (>700°C), however the net power output is curtailed by greater heat losses at the receiver and an increased spillage requirement due to diminishing AFD levels.

Comparison of different analytical models for heat and mass transfer characteristics of an evaporating meniscus in a micro-channel

Based on the augmented Young–Laplace equation and lubrication theory, a detailed analytical model predicting the heat and mass transport characteristics of the evaporating meniscus in a micro-channel is developed. The present model solves the third-order differential equation of the film thickness and the first-order differential equation of the liquid average velocity simultaneously. The different models for thermal performance of the evaporating meniscus are also compared to each other. It is found that both the film thickness and cumulative heat flux by the present model are in good agreement with the results by the previous models. The total heat flux of evaporating meniscus based on interface temperature calculated by Clausius–Clapeyron equation is far higher than that obtained by Wayner's interface mass flux model.

Observation of South Atlantic subtropical mode waters with Argo profiling float data

South Atlantic subtropical mode water (SASTMW) is detected using temperature and salinity profiles obtained by Argo floats. Marked by low potential vorticity within its extent, mode waters are identified by homogeneous temperature and salinity values, both horizontally and vertically. Their presence is evident in three regions: the Brazil Current recirculation gyre on the western side of the basin, the eastern side of the basin, and along the southern edge of the gyre. Formation occurs between winter and mid-spring in the regions of cumulative cooling of the ocean. We showed that Argo profilers do not preferentially sampled eddies. Yet, from the profiles that found SASTMW inside an eddy, 80% are anticyclonic. Furthermore, the mode water layer inside anticyclonic eddies is on average thicker than inside cyclonic eddies or anywhere else. We investigate the correlation between the presence of mode water with the eddy kinetic energy (EKE). The cross spectrum shows that the EKE and the layer thickness are out of phase in the annual period band. We addressed the role of negative surface heat fluxes during the SASTMW period of formation. Nearly all profiles that indicated mode water formation occurred at the time and within the region where the cumulative heat flux was negative.

Convective heat transfer equation for turbulent flow in tubes applied to internal combustion engines operated under motored conditions

An equation developed for the case of turbulent flow in tubes was applied for the study of convective heat transfer in internal combustion engines. Calculated average heat flux values under motored conditions were compared to measurements, results available in the literature obtained by using a computational fluid dynamics (CFD) code and the correlations of Woschni and Annand. Cumulative heat flux values for the four models were also compared to measurements at the end of the compression stroke and start of expansion. Average heat transfer rates were found to be predicted with reasonable accuracy by the proposed model. The major advantage of this equation compared to the correlations of Woschni and Annand is that it does not require any corrections for specific engine characteristics or working conditions. The proposed model was found to correctly predict higher heat transfer rates as engine speed, load and compression ratio were increased. Accuracy at the end of the compression stroke was found to be within acceptable limits for the cumulative heat flux, comparable to that of the CFD code, except for one case of the seven different engines investigated. Therefore, this model can be used to predict spatial average convective heat flux values with fairly good accuracy under motored conditions, when limited information on engine geometry is available, or when computing resources are required to be minimal.

Mixed convection in a non-Darcian fluid saturated square porous enclosure under multiple suction effect

This study examines the influence of multiple injection/suction at bottom/top walls on non-Darcy mixed convection flow in a vertical square enclosure filled with fluid saturated porous medium. The coupled non-linear partial differential equations modeling the fully developed Forchheimer extended Darcy flow are solved by finite element method. Detailed numerical simulations are carried out for a wide range of parameters such as Grashof number ( Gr∗ ∈ [0, 10]), Rayleigh Number ( Ra ∈ [50, 500]), injection/suction velocity ( a ∈ [0, 0.5]), injection/suction window width ( D/ H ∈ [0, 0.3]). The computed flow and temperature fields are analysed through streamlines, isotherms and cumulative heat flux plots. While the Nusselt number increases with increasing ( D/ H) it is found to decrease with increasing ‘ a’. Inertial forces significantly favor the heat transfer process when thermal buoyancy forces are large (i.e. for Ra > 100).

Non-Darcy Mixed Convection in a Porous Square Enclosure Under Suction/Injection Effects with a Non-Isothermal Vertical Wall

The influence of suction/injection at bottom/top or top/bottom walls on non-Darcy mixed convection flow with a sinusoidally varying temperature on the left vertical wall in a two-dimensional square porous enclosure is analyzed. The Forchheimer extended Darcy model is considered for flow equations. The fully developed equations are nondimensionalized, and then solved numerically by the Galerkin finite element method. A parametric study is carried out and the results are obtained for various values of the parameters such as the Grashof number (Gr*), Rayleigh number (Ra), suction/injection velocity (a), suction/injection window width (D/H), and amplitude (λ) of the sinusoidally varying temperature profile. The computed flow and temperature fields are visualized through streamlines, isotherms, and loal/global cumulative heat flux plots.

A MODEL FOR POST-EARTHQUAKE FIRE SPREAD CONSIDERING STRUCTURAL DAMAGE OF BUILDINGS CAUSED BY SEISMIC MOTION AND ITS PROGRESSION DUE TO HEATING OF FIRE

A physics-based model for post-earthquake fire spread was developed by modifying the prototype model previously proposed by the authors. In the new model, seismic motion and heating of fire are both considered as the causes of damage upon building members. The damage results in the change of fire behavior for individual building and criteria for building-to-building ignition. Indices representing the structural damage of a building are as follow: inclination angle of columns; falling ratio of exterior claddings; and deficit ratio of partitioning walls. Charring of wooden column gradually reduces its bearing capacity along with the decrease in its area of cross section. This may yield buckling of the column as well as collapse of the entire building. As to the falling of exterior claddings and burn-through of partitioning walls, the rates of degradation are assumed to be proportional to the cumulative heat flux received by the building members. The falling of exterior claddings increases the probability of ignition due to external heating, and the burn-through of partitioning walls allows transfer of heat and mass between neighboring rooms. For the verification of the new model, simulation of fire spread which followed 1995 Kobe Earthquake was conducted. Behavior of fire spread obtained by the numerical simulation was compared with the survey data at which reasonable agreement was obtained.

Parallel computation of natural convection in trapezoidal porous enclosures

In this study, coupled non-linear partial differential equations governing the natural convection from an isothermal wall of a trapezoidal porous enclosure have been solved numerically by finite element method (FEM) in conjunction with GMRES, a Krylov subspace based solver. In view of the enormous amount of computation, a parallel numerical algorithm for incomplete LU-conjugate gradient (ILU-CG) solver on eight-noded ANUPAM cluster under MIMD paradigm based on ANULIB message passing library has been developed. Parallel computations have been carried out for various values of flow and geometric parameters both under Darcian and non-Darcian assumptions on the porous model. Cumulative heat fluxes and Nusselt number (Nu) associated with convection process are presented through computer generated plots.

Free convection in a non-Darcian wavy porous enclosure

A numerical study of non-Darcian natural convection in a wavy vertical enclosure filled with a porous medium is performed. The flow is modelled using Forchheimer extended Darcy model. The governing equations are solved by finite element method. Numerical results demonstrate the importance of non-Darcian effect. Cumulative heat flux and Nusselt number together with flow and temperature distributions in the form of streamlines and isotherms are presented for a wide range of governing parameters like Grashof number, Rayleigh number, wave amplitude, wave phase and roughness parameter.

EFFECT OF THERMAL STRATIFICATION ON FREE CONVECTION IN A FLUID-SATURATED POROUS ENCLOSURE

Natural convection heat transfer from an isothermal vertical surface in a fluid-saturated porous medium under the influence of thermal stratification has been numerically analyzed using the finite element method. The combined effect of Rayleigh number and thermal stratification on the global heat flux in porous cavities of varying aspect ratios has been analyzed. It is observed that the global heat flux (1) decreases with increasing values of the thermal stratification b and (2) increases with increasing values of Rayleigh number Ra. However, the cumulative heat flux along the midsegment of the vertical wall increases with increasing b and decreases with Ra. With increasing b, a reduction in the convection zone with a downward drift is noticed. Global heat flux can be boosted by reducing the width L of the cavity by a factor of ∼8 or more. The flow field and the heat transfer results art presented through the streamlines, isotherms, and global heat flux plots.