Main Heat Sink Research Articles

Impacts of urban morphology on sensible heat flux and net radiation exchange

Urban morphology affects the sensible heat flux and net radiation exchange which can alter urban heat mitigation plans. This study first parameterized the geometric effects on the net radiation, and then calculated the net radiation and sensible heat flux in the urban landscape of Hong Kong. Considering that the sensible heat flux is the main heat sink in compact urban areas, this study proposes a Normalized Urban Sensible Heat Mitigation Index (NUSHMI) based on the ratio of the net radiation and sensible heat flux. Overall, there is major difference in the dependence of net radiation and sensible heat flux on geometric parameters. Net radiation Rn, reaches an optimal value, either maximum or minimum depending on the parameters of SVF and a standard deviation of building height σh, at intermediate parameter values, which suggests a guideline relevant to urban design targeting the mitigation of urban climate. Contrariwise, sensible heat flux decreases or increases, again depending on SVF and σh, is being considered, with increasing values of the same parameters. For example, Rn, reaches a minimum value for a Sky View Factor (SVF) between 0.5 and 0.6, while it reaches a maximum value for a standard deviation of building height σh between 20 and 30 m. These two results suggest that radiative forcing, i.e. Rn, can be minimized by urban space with SVF around 0.55 and σh around 25 m. The relationships between sensible heat flux and SVF or σh do not show multiple minima or maxima (as with Rn), with the exception of building density, which could also be applied as a guideline in urban design. The results based on the proposed NUSHMI indicated the NUSHMI reaches the highest values when building density is about 0.7 and building height is about 80 m and when the building height standard deviation within an area is about 10 m to 20 m. These findings revealed how the urban morphology affects the surface heat flux exchange between urban canopy and atmosphere boundary layer, and can help to design an efficient urban landscape towards urban heat mitigation for highly compacted cities, e.g. controlling the building density, height, and the height deviation. This combination of urban geometric parameters identifies an urban configuration maximizing the dissipation of absorbed radiant energy as sensible heat. It should be noted, however, that heat load upon buildings would be reduced at the price of maximizing heat dissipation within the built-up space.

Analysis of Thermal Characteristics of Airborne Integrated Environmental Control System

Taking the simplified airborne integrated thermal management system as the research object, the change characteristics of the system under different control modes are analyzed, which provides a theoretical basis for the study of system control schemes. Using a combination of mathematical models and computer models to establish an integrated airborne thermal management system model with fuel as the main heat sink, air/fuel heat exchanger, fuel/PAO heat exchanger and other main components, and A fuzzy self-tuning PID control method is proposed to analyze system characteristics such as fuel/PAO heat exchanger and electronic equipment thermal changes. The results show that compared to open-loop control under thermal extreme conditions, the fuzzy self-tuning PID control mode can not only effectively control a series of equipment such as fuel pumps to maintain proper speed, but also ensure the electronic cabin and fuel/PAO heat exchanger the outlet temperature is kept within a predetermined range to meet the design requirements of the onboard thermal management system.

Characteristics of DAC operation within integrated PtG concepts

Including direct air capture technologies (DAC) in power-to-gas (PtG) approaches offers the opportunity to close the carbon cycle by producing methane on a renewable, carbon neutral basis, while balancing fluctuations associated with an energy supply based on renewable sources. In this respect, various synergies can arise within the integrated process structure, resulting in a strong coupling of process unit operation points. Being both, carbon source and main heat sink within the system, the influence of the DAC unit on the entire process is investigated by detailed simulation and pinch analysis. Strong dependency of overall material and energy balances on DAC operation point is revealed. Slip streams of air in this unit are found to result in an increased catalyst requirement of methanation reaction by more than a factor of six, while minor decline (18%) of catalyst demand with decreasing DAC desorption temperature is observed. Use of electrolysis as heat source for DAC seems not beneficial. Overall, severe influence of DAC operation on integrated process design for autothermal production of methane based on carbon dioxide and water captured from ambient air is confirmed.

Modeling and simulation of aircraft integrated thermal management system

The integrated thermal management system for a certain type of aircraft was studied. The integrated thermal management system, which takes fuel oil as the main heat sink, thermal protection structure and liquid evaporator as auxiliary heat sink, can comprehensively control and manage the heat of the whole aircraft. The mathematical model for simulation of integrated thermal management system was established. The simulation model of the system was built based on Flowmaster software for calculation. Results show that the system proposed in this paper can meet the overall thermal requirements of the aircraft, and provide a reference for the design of the aircraft integrated thermal management system and its subsystems.

Simple Parameterization of Aerodynamic Roughness Lengths and the Turbulent Heat Fluxes at the Top of Midlatitude August‐One Glacier, Qilian Mountains, China

AbstractThe fluxes of sensible heat (H) and latent heat (LE), which are generally the important parts of the energy and mass balances over glacier surfaces, are widely quantified by the bulk method. However, due to the difficulty of determining the aerodynamic roughness length z0m in this method, H and LE values may still have large uncertainties with significant inaccuracy. To acquire reliable varying and intrinsic z0m values, new simpler parameterizations for z0m values at different ranges of the friction velocity u*b were fitted in this study. The method was implemented using the related meteorological data and glacial sublimation/condensation measured at the top of the August‐one glacier (4817 m asl) in the Qilian Mountains from 1 July 2016 to 15 August 2017. The parameterization shows that z0m increased sharply when u*b exceeded 0.43 m/s (the approximate threshold value) and that the effect of snowdrift was slight for the hourly z0m values in the range 0.15 ≤ u*b ≤ 0.43 m/s, which could thus be used to calculate the daily z0m. During the wet period (1 July to 24 September 2016 and 5 May to 15 August 2017), the turbulent fluxes calculated by the bulk method showed that net radiation Rnet was the primary source of surface energy (60.7 w/m2), whereas during the dry period (25 September 2016 to 4 May 2017), the main heat sink was the positive H (28.5 w/m2) rather than the negative Rnet (−10.0 w/m2).

Transient behavior of ASTRID with a gas power conversion system

The present article is dedicated to preliminary transient studies carried out for the analysis of the system overall behavior of the ASTRID (Advanced Sodium Technological Reactor for Industrial Demonstration) demonstrator developed in France by CEA and its industrial partners. ASTRID is foreseen to demonstrate the progress made in SFR technology at an industrial scale by qualifying innovative options, some of which still remain open in the areas requiring improvements, especially safety and operability. Among the innovative options, a gas power conversion systems (PCS) is envisaged. In this innovative PCS, the working gas is nitrogen whose flow rate delivers power to a turbine driving with the same shaft two compressors (low and high pressure) separated by an intercooler. The other part of the work delivered by the gas is used to drive the alternator that produces electricity. The main objective of such a PCS consists in avoiding physically the possibility of a sodium/water reaction with the secondary circuit but the impact of this PCS on the control of incidental and accidental transients has also been studied. The main purpose of the studies presented in the paper is to assess the dynamic behavior of ASTRID including a gas PCS with the CATHARE2 code. The first transient presented deals with a loss of off-site power and has been calculated for the gas PCS but also for a classical steam/water PCS for comparison purpose. Then typical transients of gas system have been investigated. Several families of initiating events affecting the PCS are studied: breaches in the ternary circuit, a loss of power supply, a loss of main heat sink and the spurious opening of the turbine by-pass valve. Regarding this last transient, sensitivity studies have shown that an adequate design of the turbomachine by-pass lines allows a control of the event without shutdown. Moreover, regulating actions following the scram are also investigated in order to take benefit of the PCS in order to remove the decay heat.

Torque, Power, Losses, and Heat Calculation of a Transverse Flux Reluctance Machine With Soft Magnetic Composite Materials and Disk-Shaped Rotor

In this paper, the authors introduce a type of transverse flux reluctance machines. These machines work without permanent magnets or electric rotor excitation and hold several advantages, including a high power density, high torque, and compact design. Disadvantages are a high fundamental frequency and a high torque ripple that complicates the control of the motor. The device uses soft magnetic composites (SMCs) for the magnetic circuit, which allows complex stator geometries with 3-D magnetic flux paths. The winding is made from hollow copper tubes, which also form the main heat sink of the machine by using water as a direct copper coolant. Models concerning the design and computation of the magnetic circuit, torque, and the power output are described. A crucial point in this paper is the determination of hysteresis and eddy-current losses in the SMC and the calculation of power losses and current displacement in the copper winding. These are calculated with models utilizing a combination of analytic approaches and finite-element method simulations. Finally, a thermal model based on lumped parameters is introduced, and calculated temperature rises are presented.

Thermal Design and Constraints for Heterogeneous Integrated Chip Stacks and Isolation Technology Using Air Gap and Thermal Bridge

This paper summarizes the thermal challenges in conventional 3-D stacks and proposes a novel stacking structure that eases the thermal problem. The objective of this paper is first to define limits and opportunities for developing different 3-D chip stacks from a thermal perspective, and second to explore our proposed system as a function of microbumps, through silicon vias, die thickness, and other design parameters. In our proposed 3-D stack, the interposer integrated microfluidic heat sink serves as the main heat sink. To thermally decouple stacked dice, we propose air gap isolation between them and a thermal bridge on top of the stack to cool down the isolated die. To evaluate the thermal benefits of the stack, a thermal model is developed based on the finite difference method. Several chip stack scenarios are studied and the simulations are conducted with a processor power of 74.63 W/cm 2 and memory power of 2.82 W/cm 2 . The proposed architecture yielded processor and memory temperatures of 64°C and 40°C, respectively, compared with 76°C and 75°C for the air cooled stack.

Effect of Wind on Smoldering Combustion Limits of Moist Pine Needle Beds

We studied moist pine needle beds burning under the effect of wind, in order to determine the upper moisture limit for which there is fire propagation for different wind velocities. For this purpose we built a wind tunnel that allowed us to burn a 600 mm by 150 mm by 40 mm bed under wind velocities between 0.5 m/s and 5.0 m/s and controlled air temperature. Results show an increase in moisture limit from 54% to approximately 140%, for the velocity range indicated. Combustion at limiting conditions proceeds mainly by smoldering with some periods of flaming combustion. It was observed that, for conditions close to extinction, the smoldering front is not quenched at the surface. Additionally, it was also observed that a strong flow of hot gases exit from the fuel bed at the free surface. These two observations lead to the conclusion that the main heat sink is moisture evaporation and that heat losses to the surroundings is reduced by the blowing effect of the hot gases coming off the bed. A dimensional analysis suggests a correlation between moisture limit and wind velocity of the form M = A − B/ $$ {v} $$ 2 , where M is moisture limit for fire propagation, A and B are constants, and $$ {v} $$ w is wind speed. Two dimensionless numbers helped to plot the smoldering temperature and fire propagation velocity in a more meaningful way. They are $$ \Uppi_{1} = {{T_{sml} c_{p,g} } \mathord{\left/ {\vphantom {{T_{sml} c_{p,g} } {v_{w}^{2} }}} \right. \kern-0pt} {v_{w}^{2} }} $$ and $$ \Uppi_{2} = {{Mh_{fg} } \mathord{\left/ {\vphantom {{Mh_{fg} } {v_{w}^{2} }}} \right. \kern-0pt} {v_{w}^{2} }} $$ , where T sml is smoldering temperature, c p,g is the gas specific heat, M is fuel moisture content and h fg is the latent heat of water evaporation. A relatively high moisture limit at 5 m/s wind velocity is possible due to the relatively high air flow into the smoldering front and the efficient heat feedback produced in forward smoldering.

Reassessment of external events in view of the Fukushima accident

Abstract As a consequence of the EU stress tests following the Fukushima NPP accident, the Nordic BWR Owners Group (NOG) initiated an analysis related to the capability of Swedish and Finnish BWR plants to withstand potential effects from external flooding and other extreme natural events. A review of existing analyses was performed with the intention to identify and reassess assumptions and simplifications made, and to judge whether all significant aspects of external events had been identified and considered. A previous NOG project dealing with methodologies for analyzing certain external events was used as a reference. Additional analyses were performed within three sub-projects addressing extreme water levels in the Baltic Sea and Skagerrak, methodology for the analysis of multiple external events, and main heat sink accessibility. It was concluded that the outcome from the previous NOG project can still be used as a basis, but some issues requiring further analysis were also identified.

Comparative study of the radiative and turbulent energy fluxes during summer and winter at the edge of the Antarctic ice sheet in Dronning Maud Land - East Antarctica

Present study compares the estimated radiative and turbulent energy fluxes at the edge of the Antarctic ice sheet during summer and winter in Dronning Maud land, East Antarctica. Hourly snow meteorological parameters were recorded and analysed during winter months (May, June, July and August) of the year 2007 and summer months (November, December, January and February) of the year 2007-08 using Automatic Weather Station (AWS) on the glacier surface. Snow-meteorological parameters air temperature, relative humidity, wind speed, wind direction, incoming solar radiation, outgoing solar radiation, atmospheric pressure and glacier surface temperature were recorded by the AWS. An energy balance model was used to evaluate the surface energy fluxes from measured meteorological quantities for the summer and winter. Net radiative flux was observed the main heat source during summer with seasonal average of 98Wm-2 while sensible heat flux was observed main heat source during winter with seasonal average of 30 Wm-2. Latent heat flux was observed the main heat sink during both the season with seasonal average values of -86.7 Wm-2 for summer and -65.4 Wm-2 during winter. Sublimation was observed high during summer compare to winter.

Study of surface energy and mass balance at the edge of the Antarctic ice sheet during summer in Dronning Maud Land, East Antarctica

This study estimates energy and mass balance at the edge of the Antarctic ice sheet close to a non-glaciated area. An automatic weather station was installed on the ice sheet, near an ice free area of Schirmacher Oasis in Dronning Maud Land, East Antarctica. Hourly snow-meteorological parameters were recorded and observed during the summer of the year 2007―08. Hourly radiative and turbulent energy fluxes were estimated at the ice surface. An ultrasonic sensor was used to measure accumulation or ablation at the glacier surface. Ground Penetrating Radar was also used to measure the changes in ice thickness at the observation point. The net radiative flux was the main heat source and the latent heat flux was the main heat sink for the ice sheet with seasonal average values of 98 W m -2 and -86.7 W m -2 respectively. There was a high ablation rate for the ice sheet near the non-glaciated area with a seasonal mean of 0.0172 m w.e. per day. Over the period 10 November 2007―7 February 2008 the mass balance was -1.53 m w.e. Good correlation (r 2 = 0.97) was observed between estimated and observed hourly ablation of the glacier. Sublimation and melt processes contributed 16.5% and 83.5% respectively to the net summer ablation.

A flow microcalorimeter for measuring the heats of mixing of binary liquids by A continuous process

In order to measure heats of mixing or heats of dilution for many solutions by a continuous process accurately, a twin conduction type flow microcalorimeter has been constructed, improving the design of the same type of calorimeter reported previously. The thermal stability of the heat sink was enhanced by enlarged contact area of the main heat sink block with a thermostated water bath. Mixing efficiency of the mixing cell and the stability of the base line were improved by the use of a new mixing cell having a long mixing zone and symmetrically arranged sample and reference cells. Waiting time for establishment of thermal steady state was also reduced by sandwiching each of the cell by a pair of wide thermopiles plates and a pair of sub-blocks. Heats of dilution of aqueous urea solutions were measured by a continuous process, and results obtained well agreed with accepted values by Guckeret al. The accuracy and thermal resolution in this calorimeter were estimated to be less than 0.05% and 3 μJ·s−1.

Probabilistic analysis of some safety aspects of a small reactor for district heating

Recently, various concepts of relatively small reactors for district heating have been studied in several countries as conceivable substitutes for fossil energy. Reactors of this type must fulfil very high safety requirements because they will often be sited near to regions of high population density. In this paper a small light water moderated heating reactor of 10 MW thermal power is considered. This order of magnitude is chosen for economic reasons due to the distribution of the population. The safety aspects of this reactor have been analysed using the methods of probabilistic risk analysis. The scenarios with the strongest accident potential are those initiated by a loss of the main heat sink. Our investigations were therefore concentrated on those scenarios.

Thermal properties of annular and array geometry semiconductor devices on composite heat sinks

The thermal spreading resistance and temperature-profile properties of a circular semiconductor device mounted on a composite heat sink are considered theoretically. The results are applied to annular and array-geometry contacts by the principle of superposition. It is found that considerable reductions in thermal resistance can be achieved which are due both to contact geometry, and the composite heat sink. Substantial improvements in the uniformity of the temperature profile in the contact due to the composite nature of the heat sink is also demonstrated and optimum heat sink geometries are illustrated. An annular-geometry structure (inner radius to width ratio of 8) with an area of 3·14 × 10 −4 cm 2 has a predicted thermal resistance of 2·6°C/W and a temperature difference at the interface of only 7·5 per cent, when the main heat sink is copper, the conductivity ratio is 3, and the heat sink geometry is optimum. The results are shown to agree in the special case of a single-medium heat sink with those of Frey. The results are also applicable to the case where interface layers are present in the heat sink with conductivity lower than the main heat sink and in which heat-spreading is not negligible.