Practical application of natural convective heat exchange of electrical equipment with air

This paper provides the physical and mathematical model of the air flow in a volume, containing electrical equipment with the heat-generating and heat-absorbing surfaces. The model predicts the temperature fields and air flow velocities across the volume. Using the developed model, we calculate the values of heat fluxes in the vicinity of the thermostated electrical equipment for three different cases: natural convective, forced and mixed modes of the airflow. The possibility of beneficial use of natural convective air flows for the transfer of thermal energy is analyzed. The results are applied in an industrial enterprise. Energy consumption for ventilation is significantly reduced.

Simulation of thermal processes in the lining of the ship's boiler

Судовой котлоагрегат подвергается воздействиям высокого давления рабочего тела и температуры дымовых газов. Кроме того работа его осложняется также быстрой и частой сменой нагрузки. Для продолжительной и надёжной работы котлов необходимо обеспечить прочность их конструкций, в частности, футеровки. Судовые котельные агрегаты футеруются огнеупорными материалами, во многом определяющими срок службы котла. При тепловом воздействии на теплоизоляционные материалы футеровки возникают термические напряжения, приводящие к деформации, растрескиванию и разрушению кирпичной кладки. Однако ввиду сложности постановки прямого физического эксперимента пока нет однозначного ответа на вопрос, какие условия способствуют разрушительному тепловому воздействию на футеровку котла. Потому авторы предлагают исследовать тепловые процессы в кирпичной кладке методами математического моделирования. В прикладном пакете ANSYS R17.2 WORKBENCH была создана твердотельная модель элемента футеровки (кирпича), на которой исследовались стационарные и нестационарные процессы теплообмена с граничными условиями первого и третьего рода. В результате экспериментов установлено, что разность деформаций соседних слоев огнеупора пропорциональна градиенту температуры, причём в нестационарных режимах теплообмена величина температурного градиента может значительно превышать его значение в стационарных условиях. Если учесть, что при форсированной нагрузке температура дымовых газов в топочном объёме достигает предельных значений, а интенсивность конвективного теплообмена существенно возрастает, то температурные напряжения, возникающие в футеровке котла, могут превысить предел прочности огнеупора. The ship's boiler unit is exposed to the high pressure of the working fluid and the temperature of the flue gases. The operating conditions are aggravated with rapid and frequent alternations in load. To ensure continuous and reliable operation, boiler and its elements, including lining, design needs to be strong. Ship boilers are lined with fire resistant materials, which lining basically defines lifetime of a boiler. Any heat impact to lining insulation will result in thermal stress that leads to deformation, cracking and destruction of brickwork. However, as direct physical experiment is difficult to conduct, there has been no clear understanding as to what conditions cause destructive thermal impact to the boiler lining. In light of this, the authors propose to investigate thermal processes in brickwork by mathematical modeling methods. Using ANSYS R17.2 WORKBENCH application package, the solid model of the lining element (brick) was created and stationary and non-stationary heat exchange processes with the boundary conditions of the first and third order were investigated. The experiments showed that the difference of deformations of neighboring lining layers was proportional to temperature gradient, yet in non-stationary heat exchange mode the temperature gradient can be significantly higher than that in stationary conditions. Considering that in forced loading mode the temperature of flue gases in boiler furnace can reach its limit and intensity of convective heat exchange increases significantly, the temperature stress that occurs in the boiler lining can exceed the strength of fire resistant brickwork.

Numerical study of forced convective heat exchange in spherical fillings of regular structure: determination of heat characteristics

The required operation of a heat exchanger with spherical filling depends on determining the heat characteristic of the porous medium. Using most existing dependencies to determine the heat-exchange coefficient in a porous medium gives contradictory results. This paper proposes a calculation method to determine heat characteristics of a porous medium (namely, spherical filling) using numerical modeling. This issue is addressed in a 3D setting using the RANS equation systemk- ε RNG, a turbulence model, energy and state equations for gas flows through the porous medium represented as regular packed beds of three types. Two modes are considered, which differ in the working medium type and heat exchange process direction. Modeling is done to determine the following parameters; Reh=100…4·104, Prwb=0.57… 0.919, ε=0.2595… 0.4764, Twb=300… 1900 K, Tw=300… 1900 K. The analysis results of the influence of the temperature factor, porosity, and heat physical properties of the working medium on the heat exchange are represented. Heat characteristics are given for each case scenario: about 360 reference points are obtained. A modified type of criterial dependencyNusf(Reh) is proposed and approximation coefficients are determined.

Numerical Simulations of Heat Transfer Phenomena through a Baffled Rectangular Channel

In presence of baffle, the turbulent airflow phenomena as well as forced convective heat exchange characteristics in two-dimensional rectangular channel have been analyzed in this work. For variations in Reynolds number (Re), we have studied the variations in characteristics of thermal behavior due to the change in the shape of baffle. Computations have been done using finite volume method (FVM) and FLUENT software and the SIMPLE algorithm has been employed for solving the governing equations. Finally, the flow and thermal exchange characteristics viz., streamline flow, turbulence intensity (TE), axial velocity, turbulence kinetic energy (TKE), normalized friction factor (F), normalized average Nusselt number (Nuavg) and thermal enhancement factor (TEF) have been studied in details from numerical standpoint. It has been found that the triangular shaped baffle provides highest value of F at Re = 30,000 and at Re = 46, 000, the maximum value of the TEF is found for the same baffle implying that triangular shaped baffle is more suitable for overall purposes.

Investigating the radiation-convective heat exchange of a metal-ceramic plate

The paper presents the experimental data on heat exchange during thermo-cyclic interaction of a flame jet with a thermal barrier ceramic coating. Cooling due to radiant heat transfer into the environment allows organizing heat exchange in such a way that when a flow of combustion products with a temperature of 1500 °C flows onto the sample, the temperature on the surface of the TBC does not exceed 1200 °C. The thickness of TGO after 1500 thermal cycles was 9.5 microns

Enhancement of Convection Heat Transfer in Air Using Ultrasound

The use of ultrasound is a new method to enhance convection drying. However, there is little information in the literature on the improvement of convective heat transfer caused by ultrasound. Therefore, the heat transport during ultrasound-assisted convective heating of small samples in a hybrid dryer was experimentally examined. A small Biot number regime of heat transfer was considered. The results confirmed a great enhancement of heat transfer due to the application of ultrasound. Due to the use of ultrasound, the convective heat exchange coefficient increased from 45% to almost 250%. The enhancement is a linear function of applied ultrasound power. It was shown that the energy absorption of ultrasound existed, but the thermal effect of this absorption was very small.

Analysis of Expected Skin Burns from Accepted Process Flare Heat Radiation Levels to Public Passersby

Hot flaring, even from quite high flare stacks, may result in significant heat radiation outside a facility to, e.g., public roads where random passersby may be exposed. The present study suggests a novel method for analyzing a typical flare heat radiation exposure and investigates skin burns that may be inflicted on an exposed person if a facility needs to depressurize in an emergency situation. A typical radiation field from an ignited natural gas vent was taken as the boundary condition, and these values were compared to radiation levels mentioned by the American Petroleum Institute (API 521), e.g., 1.58 kW/m2 and above. Due to facility perimeter fences along roads in larger industry areas, it was assumed that an exposed person may flee along a road rather than in the ideal direction away from the flare. It was assumed that naked skin, e.g., a bare shoulder or a bald head is exposed. The Pennes bioheat equation was numerically solved for the skin layers while the person escapes along the road. Sun radiation and convective heat exchange to the ambient air were included, and the subsequent skin injury was calculated based on the temperature development in the basal layer. Parameters affecting burn severity, such as heat radiation, solar radiation, and convective heat transfer coefficient, were analyzed. For small flares and ignited small cold vents, no skin burn would be expected for 1.58 kW/m2 or 3.16 kW/m2 maximum heat radiation at the skin surface. However, higher flare rates corresponding to, e.g., 4.0 kW/m2 maximum flare heat radiation to the skin, resulted both in higher basal layer temperatures and longer exposure time, thus increasing the damage integral significantly. It is demonstrated that the novel approach works well. In future studies, it may, e.g., be extended to cover escape through partly shielded escape routes.