Air Convective Heat Transfer Research Articles

Thermal analysis of gas turbine disk integrated with rotating heat pipes

The combination of the rotating heat pipe with conventional air cooling technique can be considered as an emerging and effective cooling technique for gas turbine disk. Accordingly, the thermal steady and transient analysis of a simplified turbine disk integrated with heat pipes have been numerically investigated. The steady and transient temperature variations in the presence and absence of heat pipes were investigated for various parameter, such as the thermal conductivity of the disk, the convective heat transfer coefficient for both the air and heat pipes, the dimension of the disk, and the number of heat pipes. The thermal analysis were performed by using finite element (FE) modeling software ANSYS-17.2. The extensive numerical simulations showed that when the number of heat pipes equal to 32, the maximum temperatures at the disk edge can be decreased by more than 100 degree. Additionally, increasing the convective heat transfer coefficient of the working fluid inside the heat pipes up to 10,000W/m2.°C, the maximum temperature at the disk rim can further be reduced by more than 280 degree. It has also been observed that the time required to achieve the minimum steady-state temperature was more sensitive to the air convective heat transfer coefficient.

Numerical modelling of convective heat transport by air flow in permafrost talus slopes

Abstract. Talus slopes are a widespread geomorphic feature in the Alps. Due to their high porosity a gravity-driven internal air circulation can be established which is forced by the gradient between external (air) and internal (talus) temperature. The thermal regime is different from the surrounding environment, leading to the occurrence of permafrost below the typical permafrost zone. This phenomenon has mainly been analysed by field studies and only few explicit numerical modelling studies exist. Numerical simulations of permafrost sometimes use parameterisations for the effects of convection but mostly neglect the influence of convective heat transfer in air on the thermal regime. In contrast, in civil engineering many studies have been carried out to investigate the thermal behaviour of blocky layers and to improve their passive cooling effect. The present study further develops and applies these concepts to model heat transfer in air flows in a natural-scale talus slope. Modelling results show that convective heat transfer has the potential to develop a significant temperature difference between the lower and the upper parts of the talus slope. A seasonally alternating chimney-effect type of circulation develops. Modelling results also show that this convective heat transfer leads to the formation of a cold reservoir in the lower part of the talus slope, which can be crucial for maintaining the frozen ground conditions despite increasing air temperatures caused by climate change.

Heat dissipation from a stationary brake disc, Part 1: Analytical modelling and experimental investigations

The main aim of the research is to support the development of the commercial vehicle electric parking brake. Though nowadays widely used on passenger cars, electric parking brake applications on commercial vehicles present completely different challenges. With the brake mass, thermal capacity and required clamp forces an order of magnitude higher, safe parking demands much more attention. In the first instance, the priority is placed upon predicting heat dissipation from the brake disc only. The research is presented in two parts; part one (presented here) focuses on analytical modelling and experimental verification of predicted disc temperatures over long cooling periods, with part two investigating the air flow, velocities and convective heat transfer coefficients using computational fluid dynamics modelling, also followed by experimental validations. To begin the analytical analysis, a study was conducted into the variance in mean local convective heat transfer coefficients over a simplified brake disc friction surface, by investigating typical dimensionless air properties. A nonlinear equation was derived for the average surface convective heat transfer coefficient ([Formula: see text]) variability with temperature drop for the entire cooling phase. Starting from fundamental principles, first-order differential equations were developed to predict the bulk disc temperature. By including variation of the convective and radiative heat dissipation throughout the cooling period, a good correlation was achieved with measured values, to within 10%. Experiments were conducted on a specifically designed thermal rig which uses 15 kW induction heater to heat the disc. Numerous experiments proved the results are very repeatable, throughout the cooling period. It was established, for the grey cast iron brake disc with a fully oxidised surface, the emissivity value are practically constant at ɛ = 0.92. Although the research is being conducted on a brake disc, the results have generic application to any disc geometry, whatever the application.

Quantify Impacts of Local Urban Microclimate on Local Airflow Patterns

This study aims to quantify the impacts of the local urban microclimate on the local airflow patterns, including air temperature and Convective Heat Transfer Coefficients (CHTCs). Consideration of the microclimate require taking into account influence of the local outdoor air temperature, velocity, and solar irradiance. Accurate representation of these variables plays an important role in the assessment of building energy consumption patterns in urban areas. This study uses Computational Fluid Dynamics (CFD) with implementing the local solar irradiances on the building surfaces to calculate local airflow patterns. The performed simulations indicate accounting for the local urban microclimate allows potential implementation of urban scale CHTCs in the building energy simulation models. Accurate estimation of local microclimate provides crucial inputs for the building energy models.

What is the Required Level of Details to Represent the Impact of the Built Environment on Energy Demand?

A full coupling between a CFD code, a thermo-radiative model and a building energy simulation model enables Solene-microclimat software to calculate both building thermal behavior and urban microclimate with the retroaction of buildings on microclimate. However, this full coupling is time consuming and it is legitimate to wonder if it is always necessary to perform such detailed simulations. In the framework of the MERUBBI project, simulations were carried out to answer this question. A set of simulations was designed to explore different kinds of configurations: three cities in France (Nantes, Paris and Strasbourg), three levels of density (from an isolated building to an implementation in the dense city center) and three kinds of buildings (an individual house in Paris, a residential building in Nantes and an office building in Strasbourg). To study the sensitivity of energy demand to the coupling detail, for each thermal flux at the external surfaces of the building, several levels of details were taken into account. For the impact of wind on convection, three modalities were considered: a constant convective heat transfer coefficient, calculated from the wind velocity at 10m; a convective heat transfer coefficient calculated from a vertical wind profile; a convective heat transfer coefficient calculated from the local wind velocity simulated with a CFD code. For the impact of air temperature on convection, two modalities are considered the use of the temperature measured at the nearest meteorological station; a local temperature calculated with the CFD simulation. For the impact of long-wave radiative exchanges, three modalities: the building exchanges with the sky without taking into account the masks of the environment and the long-wave radiative exchanges with the other surfaces; the building exchanges with the sky, taking into account the mask effects but not the exchanges with the surrounding surfaces; long-wave exchanges are taken into account with all kinds of surfaces in function of view factors. For the impact of short-wave radiations, two modalities: only direct and diffuse solar fluxes are taken into account; inter-reflections are considered. The results indicate that if the calculation of air temperature and convective heat transfer coefficient have few impacts in all the cases, the way of calculating long-wave and short wave radioactive fluxes has to be carefully considered, in winter as in summer. More detailed recommendations are given according to the density of the site in which the building will be implemented.

Optical and thermal modeling of a photovoltaic module and experimental evaluation of the modeling performance

In the present study, coupled optical and thermal models were used to predict the temperature distribution of photovoltaic (PV) module's layers. To verify the modeling performance, an experimental measurement was also conducted in this study and temperatures of two PV modules fixed at 10° and 30° tilt angles were measured and compared with the predicted module temperatures. Two extra experimental data sets measured in two other locations of the globe were also used to further verify the modeling performance. The modeling prediction was also compared with the results of other mathematical models developed in the past studies. This study paid a special attention to the prediction of air convection heat transfer coefficient over the PV module as well as the optical and thermal modeling. The results of this study showed that the present modeling predicts the PV module temperature with a reasonable accuracy compared to the experimental data measured in this study and the other data sets measured in the other parts of the globe. The results also showed that the present modeling predicts the PV module temperature with a higher accuracy compared with the other mathematical models developed in the past studies. Furthermore, this study showed that the accurate prediction of convection heat transfer coefficient over the PV module based on ambient temperature and wind speed, yields to an accurate prediction of PV module temperature © 2016 American Institute of Chemical Engineers Environ Prog, 36: 277–293, 2017

Experimental study of air-cooled water condensation in slightly inclined circular tube using infrared temperature measurement technique

This study presents the results of an investigation of the air-cooled water condensation heat transfer characteristics inside a slightly inclined circular tube made of transparent Pyrex glass. The high-resolution wall temperature data and stratified film formations could be obtained with the assistance of an infrared (IR) thermometry technique and side-view visualization using a CCD camera. In all experimental cases, the condensation flow patterns were in the fully-stratified flow region. In addition, the experimentally measured void fraction corresponded well with the logarithmic mean void fraction model. The local temperature differences in the cooling air flow across the condenser tube and high-resolution temperature profiles on the tube’s outer wall were obtained in the experimental measurements. Under the experimental conditions of this study, the local heat flux distributions in the longitudinal direction of the test tube were strongly dependent on the cooling air velocity. And, with the help of IR thermometry, the tube outer wall temperature data at 45 local points could be measured. From the data, the asymmetry distribution of the local wall temperatures and the accurate location of the transition from two-phase mixture to single phase liquid inside the tube could be obtained. Also, the analysis of the thermal resistances by condensation, wall conduction and air convection showed that the air convective heat transfer behavior can play a dominant role to the local heat transfer characteristics. Finally, in order to obtain the local heat flux distribution along the tube’s outer wall, a two-dimensional computational fluid dynamics analysis was conducted. From this calculation, it was resulted that about 70% of total heat transfer rate from the tube surface was released through the tube region lower than the center point. These results were caused by the distribution of air convective heat transfer coefficient on the tube surface.

Experimental and CFD investigation of convection heat transfer in solar air heater with reverse L-shaped ribs

A solar air heater is a thermal system which uses artificial roughness in the form of repeated ribs on the absorber plate to enhance the heat transfer rate. Forced convection heat transfer of air in a solar air heater with reverse L-shaped ribs has been carried out experimentally and numerically. Thermal performance of solar air heater is studied with design variables such as relative roughness pitch (7.14⩽P/e⩽17.86), Reynolds number (3800⩽Re⩽18,000), heat flux (1000W/m2) and constant relative roughness height (e/D=0.042). A two dimensional CFD simulation is carried out with using CFD code, ANSYS FLUENT and RNG k–ɛ turbulence model, for solving turbulence terms in governing equations. The presence of reverse L-shaped rib shows a significant effect on heat transfer and friction factor characteristics, relative to change in relative roughness pitch (P/e) and Reynolds number (Re). Thermo hydraulic performance parameter (T.H.P.P) of 1.90 considering heat transfer augmentation with same pumping power, has been evaluated for optimum configuration of the roughness element (reverse L-shaped rib) for artificially roughened solar air heater. It has been found that the numerical results are in good agreement with the experimental results for the range of parameters investigated. Correlations for Nusselt number and friction factor have been developed as a function of roughness and flow parameters.

Study on convective heat transfer from pig models by CFD in a virtual wind tunnel

Heat stress strongly affects the pig production in hot climate regions, including the summer of many continental climate regions. Although it is recognized that increasing the local air speed in the animal occupied zone is one of the effective approaches to decrease the heat stress of pigs, few studies have been found to quantify the relationship of air speed and convective heat transfer from the pigs. The main objective of this study is to evaluate the impact of airflow speed, turbulence intensity, and animal orientation on the convective heat transfer so that it can be implemented into ventilation control algorithms to provide a better production environment. The study was mainly performed by CFD simulations in two geometry models, a pig model with actual size and a simplified cylinder model, in virtual wind tunnel. The validation of CFD modelling was conducted by comparing the simulated Nusselt number with the values determined by semi-experimental equation for cylinder geometry model. The effects of air speed, inlet turbulence intensity, air incidence angles and pig skin modelling on convective heat transfer coefficients were studied. It is proved that CFD method could be a very good alternative way for this kind of study. The convection heat transfer of a pig model has similar tendency with the cylindrical model following the increasing of air speed. A correlation of convective heat transfer coefficient was found between the geometry of actual pig model and cylinder model. The orientation and turbulence intensity significantly affect the convective heat transfer coefficient of pig, and the results indicate animal orientation and turbulence intensity need to be considered for model development process of the convective heat transfer of pig.

Design and Optimization of PMSM by Using the Thermal Behaviour of Fluid Dynamics

One of the efficient gadgets possessing magnificent scope for the enhancement of energy efficiency is the electrical motor. Amid, several kinds of electrical motors, Permanent magnet synchronous motors are widely employed in countless sophisticated applications. This phenomenon has enabled it to be shortlisted for our investigation, which predominantly centres on the thermal character of the Permanent Magnet Synchronous Motor (PMSM), which is designed and replicated in a Computational Fluid Dynamics Software ANSYS FLUENT-13. The heat transfer inside the motor is assessed for diverse functional constraints like inlet velocity, ambient temperature, air gap thickness, heat flux, and convective heat transfer coefficient. The optimum conditions to be preserved to perk up the heat dissipation pace and to dwindle down the highest temperature are forecast. It is pertinent to note that the forecasts from the analyses closely resemble those gathered from literary community, thereby authenticating the aptness of the assessment.

Evaluating the cooling performance of crushed-rock interlayer embankments with unperforated and perforated ventilation ducts in permafrost regions

The crushed-rock interlayer embankment with ventilation ducts has been advocated to stabilize the permafrost stratum under expressways in permafrost regions. This embankment must render better cooling effect than railway embankments because the expressway surface is wider and hotter. To this purpose, the walls of the ventilation ducts need to be perforated. This study evaluates the cooling performance of the rushed-rock interlayer embankments with unperforated and perforated ventilation ducts along an expressway in permafrost regions of the Qinghai-Tibet Plateau. A three-dimensional numerical model is developed based on heat and mass transfer theories. The model includes the coupled heat transfer between air and ventilation duct wall, the air convective heat transfer in crushed-rock layer, and the heat conduction with phase change in soil layers. The numerical results indicate that if the ventilation ducts are perforated and embedded at the top of the crushed-rock interlayer, the cooling effect of the embankment can be greatly enhanced. A good cooling performance can still be achieved even if the centerline spacing of the perforated ventilation ducts is enlarged to 4 m to facilitate the construction. The crushed-rock interlayer embankment with perforated ventilation ducts is a better candidate structure for expressways in permafrost regions of the Qinghai-Tibet Plateau.

Simulations of natural convection heat transfer in an enclosure at different Rayleigh number using lattice Boltzmann method

Natural convection heat transfer of air for turbulent flow in cavity is investigated using the lattice Boltzmann method (LBM). This study appraises existing scaling laws in previous studies and covers a range of Rayleigh number (Ra) between 105 and 109. It is found that the comparison of the results at low Ra excellently agrees with the results from the literature. The dimensionless distance of heated source in cavity is one of the important factors. The results also explain the mechanism of natural convection rate increasing with the Ra. When the dimensionless length of heat source is equal to 1/5 at Ra of 108, the flow in the enclosure still keeps soft turbulent convection. The flow in cavity becomes fully turbulent natural convection at Ra=109. The core region scaling seems to approach the behavior of a passive scalar as Ra increases, i.e. it changes from pure exponential to a stretched exponential scaling. It is noticed that the results obtained from the present LBM simulations are in good agreement with the Kolmogorov-law and Bolgiano-law.

An experimental investigation of heat transfer characteristics for steam cooling and air cooling in a rectangular channel roughened with parallel ribs

A comparative experimental study on the heat transfer characteristics of steam and air flow in rectangular channels roughened with parallel ribs was conducted with the use of an infrared camera. The effects of Reynolds numbers (Re), rib spacing ratio (P/e) and rib angles (α) on steam and air convective heat transfer were obtained and compared. The blockage ratio (e/Dh) of the tested channel is 0.078 and the aspect ratio (W/H) is fixed at 3.0. In the paper, the Reynolds number for both coolants ranges from 3000 to 15,000, the rib spacing ratios were 8, 10 and 12, and rib angles were 90°, 75°, 60°, and 45°, respectively. For a square channel with 90° ribs with wider rib spacing ratio (P/e=10 and 12) respectively give a area-averaged heat transfer about 8.4% and 11.4% more than P/e=8 model. The heat transfer enhancement of both steam and air increased with decreasing the rib angle from 90° to 45°. Although the heat transfer distributions of steam and air flow in the rib roughness channels were similar to each other in all the investigated channels, the steam flow can obtain a high convective heat transfer enhancement capability. The area-averaged Nusselt numbers of steam flow at a Reynolds number of 12,000 were approximately 13.9%, 14.2%, 19.9%, and 23.9% higher than those of air flow for rib angles of 90°, 75°, 60°, and 45°, respectively.

Computer simulation of processes in the dead–end furnace

We study turbulent combustion of natural gas in the reverse flame of fire–tube boiler simulated with the ANSYS Fluent 12.1.4 engineering simulation software. Aerodynamic structure and volumetric pressure fields of the flame were calculated. The results are presented in graphical form. The effect of the twist parameter for a drag coefficient of dead–end furnace was estimated. Finite element method was used for simulating the following processes: the combustion of methane in air oxygen, radiant and convective heat transfer, turbulence. Complete geometric model of the dead–end furnace based on boiler drawings was considered.

Enhancement of heat transfer in unsteady laminar oil flow past a heated cylinder at Re = 150

Unsteady convective heat transfer in air and oil flows past a heated circular cylinder is modeled numerically by solving the unsteady Navier—Stokes and energy equations with the aid of multiblock computational technologies implemented in the VP2/3 code using composite overlapping structured grids of different topology. Enhancement of heat and momentum transfer is associated with a considerable reduction of temperature boundary layer thickness. Main attention is paid to a self-similar flow regime and heat transfer, with the analysis of averaged and fluctuation characteristics and the comparison of constant physical property and inhomogeneous media.

Simulation Investigation on Structural Parameter of Hot Air Igniter of Biomass

In view of rich resources of corn stalks, cotton stalks forming pellets hot ignition performance, using CFD numerical simulation analysis to study the key parameters of electric ignition such as the surface ignition temperature, air velocity, air flow distribution ratio of influence of hot air outlet temperature. Obtained that increasing electric heating surface temperature, reducing the air flow velocity and convective heat transfer coefficient of the outer tube can significantly improve the ignition temperature of the outlet hot air, the outlet hot air temperature should be higher than 450 °C. Ignition internal and external air passage area ratio affects the allocation of hot air flow, the best area ratio of between 1.8-2.2.

Effect of Coil Inclination on Thermomagnetic Convection of Air in a Porous Cubic Enclosure Under Microgravity Environment

The thermomagnetic convection of air in a porous cubic enclosure with an electric coil inclined in general orientations was numerically investigated under microgravity environment. Biot-Savart’s was used to calculate magnetic field. The governing equations in primitive variables were discretized by the finite-volume method and solved by the SIMPLE algorithm. The Darcy model was used to solve the momentum equations. The results show that both the magnetic force and the coil inclination have significant effect on the flow field and heat transfer in a porous cubic enclosure; the natural convection heat transfer of air can be enhanced or controlled by applying gradient magnetic field

Magnetic and Gravitational Convection of Air in a Porous Cubic Enclosure with a Coil Inclined Around the Y Axis

The natural convection heat transfer of air in a porous media can be controlled by gradient magnetic field. Thermomagnetic convection of air in a porous cubic enclosure with an electric coil inclined around the \(Y\) axis was numerically investigated. The Biot–Savart law was used to calculate the magnetic field. The governing equations in primitive variables were discretized by the finite-volume method and solved by the SIMPLE algorithm. The flow and temperature fields for the air natural convection were presented and the mean Nusselt number on the hot wall was calculated and compared. The results show that both the magnetic force and coil inclination have significant effect on the flow field and heat transfer in a porous cubic enclosure, the natural convection heat transfer of air can be enhanced or controlled by applying gradient magnetic field.

Numerical simulation and experimental investigation of natural convection heat transfer of molten salt around fine wire

In order to get the natural convection heat transfer mechanism of molten salt, the experimental investigation of natural convective heat transfer of LiNO3 was studied after it was simulated by numerical calculation. Experiment was carried out on the natural convection heat transfer of air and water around the fine wire using the method of Joule heating. The results showed that the natural convection heat transfer of air and water around the fine wire agreed well with Fand’s correlation. Based on the aforementioned experiment, the natural convection heat transfer of molten salt LiNO3 was studied by experiment and the same results were got. Therefore, the natural convection heat transfer of molten salt can be calculated by Fand’s correlation, which takes into consideration the effect of viscosity dissipation.

CFD Simulation and Experimental Investigation of Convection Heat Transfer in a Rectangular Convergent Channel with Staggered Ribs

Forced convection heat transfer of air in a rectangular convergent channel with 90˚ ribs on bottom wall was investigated experimentally and numerically. The copper test section was inlet 100 mm×80 mm×5 mm and outlet 100 mm×74 mm×5 mm convergent rectangular channel with ribs were 3,6 and 9 mm square with 60 mm pitch distance for each 4 numbers of ribs were attached in the bottom surface of the test channel. The tests investigated the effect of air mass flow rate on the convection heat transfer enhancement with the above sized ribs. Comparisons between the experimental and numerical results showed that the RNG k−e turbulence model. The numerical model was created as per the same dimension of experimental test section channel size and three different sized ribs are mounted at the bottom surface of the test section. The experimental investigation and numerical analysis was conducted for the Reynolds number 20,000. From the experimental and numerical results indicate that the heat transfer coefficients for 6mm sized ribbed channel were largest among the three sized ribs.