Heat Transfer Characteristics Of Air Research Articles

Flow and heat transfer characteristics of air and n-decane in eccentric tube-in-tube helically coiled heat exchangers

Present work proposed an eccentric tube-in-tube helically coiled heat exchanger (TTHC), used in Aero engine turbine film cooling which can precool compressed air more efficiently. The supercritical n-decane (3 MPa) at inner tube is used to precool the compressed air at annulus channel. The effects of mass flow rate, eccentric angle (ϕ) and eccentric ratio (e) of the inner tube on flow and heat transfer performance are investigated systematically by the RNG k−εnumerical model. The results showed that the Nusselt number (Nua) of concentric and eccentric TTHC (ϕ = 0°, e = 0.4) was 1.15 and 1.19 times of that for straight concentric tube-in-tube heat exchanger, respectively. The synergy effect at annulus side for the eccentric TTHC is more strengthened than the concentric one. When eccentric angle(ϕ) increased from 0° to 180° (e = 0.4), the Nua and fa decreased by 20% and 3%, and the performance evaluation factor (PEFa) decreased 10%. Moreover, when eccentric ratio (e) increased from 0 to 0.8 (ϕ = 0°), the Nua and the PEFa increased by 10.7% and 14%. Therefore, the optimal geometric parameters in eccentric TTHC can be selected as ϕ = 0°, e = 0.8.

Numerical investigation of heat transfer characteristics of high-speed and high-temperature air cooled open-cycle reactor

The nuclear aircraft is one important type in the application of near space. The high-speed and high-temperature air flows through the open-cycle reactor of the aircraft engine differs from typical reactors. The heat transfer characteristics of high-speed air flow have great effects on the reactor performance. Therefore, the simulation of the air flow in the reactor coolant channel is conducted. Firstly, the numerical methods are validated against experiments. Subsequently, the effects of the distribution of the heat flux, inlet velocity, temperature and pressure on the heat transfer performance of the coolant channel are investigated. The results reveal that the heat transfer correlations evaluated at average bulk temperature are more appropriate compared to that evaluated at the average film or wall temperature. The heat flux influences the heat transfer coefficient indirectly due to the varied air properties. The higher inlet air temperature means the larger thermal conductivity, which leads to the higher heat transfer coefficient. The heat transfer coefficient of air under high-pressure conditions can be equivalent to that of water under the forced circulation conditions. The research can lay basis for the analysis of the flow and heat transfer characteristics of the air flow in the open-cycle reactor.

Numerical and Experimental Study on the Air Convective Heat Transfer Characteristics in Large-Scale Rectangular Channel with Asymmetric Heating Considering the Influence of Entry Form

In the passive containment cooling system (PCCS) of an AP1000-type pressurized water reactor nuclear power plant, the air convective heat transfer in the external channel of the containment plays a key role under accident conditions. In this study, the containment external channel is simplified into a large-scale rectangular channel with asymmetric heating, and an experimental and numerical study is conducted to evaluate the air heat transfer characteristics. In addition, the effect of the entry form, which comprises an irregular inlet duct and a pore plate, is considered. First, the results show that heat transfer is enhanced by the entry form, which can be verified by numerical simulation and experiment. Second, when analyzing the air heat transfer characteristics of the actual PCCS annular channel, the Gnielinski correlation is relatively conservative since this correlation hugely underestimates the heat transfer rate of numerical cases in a large-scale rectangular channel with the entry form.

Study on Exhaust Air Heat Transfer Characteristics of Heat Exchange System for White Smoke Reduction

Study on Exhaust Air Heat Transfer Characteristics of Heat Exchange System for White Smoke Reduction

Heat transfer characteristics of coconut oil as phase change material to room cooling application

Thermal comfort in a room is one of human needs in the workplace and dwellings, so that the use of air conditioning system in tropical countries is inevitable. This equipment has an impact on the increase of energy consumption. One method of minimizing the energy use is by using the phase change material (PCM) as thermal energy storage. This material utilizes the temperature difference between day and night for the storage and release of thermal energy. PCM development on application as a material for air cooling inlet, partitioning and interior needs to be supported by the study of heat transfer characteristics when PCM absorbs heat from ambient temperature. This study was conducted to determine the heat transfer characteristics on coconut oil as a phase change material. There are three models of experiments performed in this research. Firstly, an experiment was conducted to analyze the time that was needed by material to phase change by varying the temperature. The second experiment analyzed the heat transfer characteristics of air to PCM naturally convection. The third experiment analyzed the forced convection heat transfer on the surface of the PCM container by varying the air velocity. The data of experimental showed that, increasing ambient air temperature resulted in shorter time for phase change. At temperatures of 30°C, the time for phase change of PCM with the thickness of 8 cm was 1700 min, and it was stable at temperatures of 27°C. Increasing air temperature accelerated the phase change in the material. While for the forced convection heat transfer, PCM could reduce the air temperature in the range of 30 to 35°C at about 1 to 2°C, with a velocity of 1-3 m/s.

경사진 원형관에서의 공냉응축에 관한 실험적 연구

In this study, the experimental investigation of air-cooled condensation in slightly inclined circular tubes with and without fins has been conducted. In order to assess the effects of the essential parameters, variable air velocities and steam mass flow rates were given to the test section. The heat transfer performance of air-cooled condensation were dominantly affected by the air velocity, however, the increase of the steam mass flow rate gave relatively weaker effects to total heat transfer capability. And in the experimental cases with the finned tube, the total heat transfer rate of the finned tube was significantly larger than that of the flat tube. From those results, it can be confirmed that the most important parameter for air-cooled condensation heat transfer is the convective heat transfer characteristics of air. Therefore, for the well-designed long-term cooling passive safety system, the consideration of the optimal design of the fin geometry is needed, and the experimental and numerical validations of the heat transfer capability of the finned tube would be required.

Effects of Heating Location and Size on Natural Convection in Partially Heated Open-Ended Enclosure by Using Lattice Boltzmann Method

Natural convection heat transfer in an square enclosure, consisting of a partially heated west wall with east end open to ambient, is investigated numerically by using an in-house computational fluid dynamics solver based on thermal lattice Boltzmann method. In particular, the influences of Rayleigh number (103–106), heating location (bottom, middle, and top) on west wall, and dimensionless heating length (0.25–0.75) on momentum and heat transfer characteristics of air are presented and discussed. The streamline patterns show bifurcation at the lowest Rayleigh number for bottom and middle heating, whereas at the highest Rayleigh number, all heating positions yield bifurcation and elongation of flow patterns with a secondary vortex near the lower side of open end. The middle and bottom heating locations show a linear increase in Nusselt number with heater size, whereas inverse dependence is seen for top heating. The maximum heat transfer is observed in the case of middle heating. As expected, average Nusselt number increased with increasing Rayleigh number. Finally, the functional dependence of the average Nusselt number on flow governing parameters (Rayleigh number and heating length) for different heating locations is presented as a simple predictive empirical relationship.

Thermal management of cylindrical power battery module for extending the life of new energy electric vehicles

Thermal management especially cooling plays an important role in power battery modules for electric vehicles. In order to comprehensively understand the heat transfer characteristics of air cooling system, the air cooling numerical simulation battery models for cylindrical lithium-ion power battery pack were established in this paper, and a detailed parametric investigation was undertaken to study effects of different ventilation types and velocities, gap spacing between neighbor batteries, temperatures of environment and entrance air, amount of single row cells and battery diameter on the thermal management performance of battery pack. The results showed that the local temperature difference increased firstly and then decreased with the increase of wind speed. Reversing the air flow direction between adjacent rows is not necessarily appropriate and the gap spacing should not be too small and too large. It is prone to thermal runaway when the ambient temperature is too high, and the most suitable value of S/D (the ratio of spacing distance between neighbor cells and cell diameter) is gradually reduced along with the increase of cell diameter.

Forced Convection Heat Transfer Analysis through Dimpled Surfaces with Different Arrangements

Dimples play a very important role in heat transfer enhancement of electronic cooling systems, heat exchangers etc. This work mainly deals with the experimental investigation of forced convection heat transfer over circular shaped dimples of different diameters on a flat copper plate under external laminar flow conditions. Experimental measurements on heat transfer characteristics of air (with various inlet flow rates) on a flat plate with dimples were conducted. From the obtained results, it was observed that the heat transfer coefficient and Nusselt number were high for the copper plate in which the diameter of dimples increases centrally in the direction of flow (case c) as compared to the other two cases.

Flow and heat transfer characteristics of ambient air condensation on a horizontal cryogenic tube

Ambient air condensation on a cryogenic horizontal tube is investigated using a newly built mathematical model, in which the liquid film and the vapor boundary layer are coupled together with a major emphasis on the effect of buoyancy. Based on the model, the heat transfer coefficients and the film thickness as well as the interfacial shear are obtained under different conditions to investigate the effects on the flow and heat transfer characteristics of the superheating between vapor and film, the buoyancy in the boundary layer and the subcooling between wall and film. In addition to the flow and heat transfer characteristics of air, the other four different vapors, i.e. H2O, R134a, methane (CH4), argon (Ar), are also discussed. The results show that the superheating has a more significant contribution to the increase of heat transfer coefficient for air comparing to the other vapors, e.g. in the cases of superheating ΔT∞=100 and 200K the mean heat transfer coefficient increases by 10.3% and 24.3% for air, while it increases by only 1.8% and 3.9% for H2O. In contrast to superheating, the subcooling has a negative effect on the increase of heat transfer coefficient. Noteworthy, the buoyancy plays a non-negligible role on the flow and heat transfer characteristics of superheated air condensation. The analytical results are of great importance in the design and improvement of ambient-heated cryogenic vaporizer (AHCV).

Analysis of Flow and Heat Transfer Characteristics Around an Oval-Shaped Cylinder

Numerical simulations and experimental study were carried out to investigate the flow and heat transfer characteristics of air flowing across different types of oval-shaped cylinders, for Reynolds numbers varying from 4000 to 50,000. These cylinders have axis ratios, ε, of 1, 1.5, 2, 3, 4, and 5 with the major axis parallel to the free-stream. Numerical results show the closer the distance to mainstream, the smaller the local velocity gradient is. The angular position of the minimum value of Cp decreases as ε decreases and the maximum value of Cf gradually increases with ε increasing. Oval-shaped cylinders have a higher favorable pressure gradient at the front of the cylinder and a lower adverse pressure gradient at the back of the cylinder for flows in inhibiting separation. Empirical correlations for each tube have been obtained by numerical simulation relating the dimensionless heat transfer coefficient with the Reynolds Number and Prandtl Number. Based on the presented results, it can be emphasized that the average heat transfer coefficient firstly increases and then decreases by increasing the axis ratio of the tube, implying that the elliptical tubes with a suitable axis ratio possess more advantages over circular tubes. Comparisons of the numerical results with the existing data verify the validation of the present study.

Heat transfer enhancement of an electric air heating furnace by inserting silicon carbide ceramic foam panels

Silicon carbide ceramic foams are used to achieve high performances in many industrial heat exchange systems. This paper experimentally investigated the heat transfer characteristics of air flowing through high-temperature silicon carbide ceramic foams in an electric air heating furnace heated by resistance wires and silicon–carbon sticks. It was found that for an air inlet flow rate of 200m3/h, the air outlet temperature reached 981°C after about 2h when five silicon carbide ceramic foam panels were inserted inside the furnace, while it only reached about 650°C when no ceramic foam was inserted. The heat transfer enhancement was due to that the ceramic foams enlarged the heat transfer area between air and hot solid surfaces. The results also showed that the position of the ceramic foam in the furnace played an important role in its effect on the heat transfer. This study well validated that silicon carbide ceramic foam can be used as radiant energy absorbers and heat exchangers especially in volumetric solar receivers in concentrated solar power (CSP) plants.

Convective heat transfer and pressure drop of annular tubes with three different internal longitudinal fins

AbstractPressure drop and heat transfer characteristics of air in three annular tubes with different internal longitudinal fins were investigated experimentally at uniform wall heat flux. The tested tubes have a double‐pipe structure with the inner blocked tube as an insertion. Three different kinds of fins, plain rectangle fin, plain rectangle fin with periodical ridges and wave‐like fin, were located peripherally in the annulus. The friction factor and Nusselt number can be corrected by a power‐law correction in the Reynolds number range tested. It was found that the tube with periodical ridges on the plain fin or with wave‐like fin could augment heat transfer; however, the pressure drop was increased simultaneously. In order to evaluate the comprehensive heat transfer characteristics of the tested tubes, two criteria for evaluating the comprehensive thermal performance of tested tubes were adopted. They are: 1) evaluating the comprehensive heat transfer performance under three conditions: identical mass flow, identical pumping power, and identical pressure drop; 2) the second law of thermodynamics, i.e., the entropy generation. According to the two different evaluating methods, it was found that the tube with wave‐like fins provided the most excellent comprehensive heat transfer performance among the three tubes, especially when it was used under higher Reynolds number conditions. © 2007 Wiley Periodicals, Inc. Heat Trans Asian Res, 37(1): 29–40, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.20186

Convection heat transfer enhancement of air in a rectangular duct by application of a magnetic quadrupole field

Air is subjected to a magnetic body force in a magnetic gradient field and can be driven to flow by magnetic buoyancy force. The laminar flow and heat transfer characteristics of air in a rectangular duct confined to a magnetic quadrupole field were numerically investigated. Calculations were performed for both hydraulically and thermally developing and fully developed laminar flows at Reynolds numbers between 200 and 2000. The centrifugal-form magnetic body force can induce air flows in the spanwise direction, and form eight longitudinal vortices. The velocity and temperature fields were presented and the friction coefficients and Nusselt numbers were compared for different Reynolds numbers and duct wall temperatures. The results show that the convection heat transfer is remarkably enhanced at the cost of a relatively slight power loss by application of the magnetic quadrupole field. For hydraulically and thermally fully developed flows, the enhancement of convection heat transfer due to the longitudinal vortices resulting from the magnetic quadrupole field is more effective.

Numerical Study on Design Parameters of a Channel with Offset Plates Heated by Radiation, Based on Maximum Forced Convection Heat Transfer Coefficient and Minimum Friction Factor

The present numerical analysis will deal with the laminar flow forced-convection heat transfer characteristics of air flowing through a channel with offset plates. Particularly, when this configuration is neither subjected to constant heat flux or having isothermal condition but mainly heated by radiation heat flux. The application of this design can be utilized in air heater solar collectors and/ or combined photovoltaic and air heater solar collector systems.

Natural Convection Between Two Horizontal Cylinders in an Adiabatic Circular Enclosure

A numerical study of natural convection flow structure and heat transfer has been undertaken for air around two horizontal, differentially heated cylinders confined to an adiabatic circular enclosure. Parametric simulations were performed to assess the effects of gap width between cylinders as well as the inclination angle of the enclosure with respect to gravity. Results clearly indicate that the fluid flow complexity and heat transfer characteristics of air amid the cylinders and enclosure wall are strongly affected by the Rayleigh number, the inclination angle, and the gap width between the cylinders. With the exception of the vertical orientation, heat exchange between the differentially heated cylinders is predominantly controlled by a counterclockwise recirculating flow enclosing them. In addition, flow visualization experiments were conducted for the physical configuration under consideration, and a generally good agreement for the flow pattern was observed between the predictions and the experiments, further validating the present numerical simulation.

Laminar mixed convection in a horizontal channel with protruded heat sources

Abstract The problem of laminar mixed convection in a horizontal dimensional channel with one or two protruded heat sources on one of the walls is studied numerically. The top surface of the protruding heat source is assumed to be isothermal while the rest of the surfaces and the walls are assumed to be adiabatic. The governing dimensionless equations in terms of vorticity, stream function and temperature are solved numerically by the cubic spline collocation method. The effects of the mixed convection parameter, Gr/Re 2, height of thermal sources, D/H, and the interaction of the protruded heat sources on fluid and heat transfer characteristics of air in a horizontal channel are presented. The results show that the heat transfer rates increase as the Gr/Re 2 and D/H are increased because of the increase of the buoyancy effects. The results also show that the effects of the interaction of the two protruding heat sources are remarkably affected by the flow field and temperature field in the horizontal channel.

Finite element solutions of laminar flow and heat transfer of air in a staggered and an in-line tube bank

A finite element method is used to solve the full Navier-Stokes and energy equations for the problems of laminar flow and heat transfer characteristics of air around three isothermal heated horizontal cylinders in a staggered tube bank and around four isothermal heated horizontal cylinders in an in line tube bank. The variations of surface shear stress, pressure and Nusselt number are obtained over the entire cylinder surface, including the zone beyond the separation point. The predicted values of total drag, pressure drag and friction drag coefficients, average Nusselt number, and the plots of velocity flow fields and isotherms are also presented.

Calculated Heat Transfer Characteristics of Air and SF6

This paper reviews the basic theory of convection and radiation in gases and presents general formulae which can be used to calculate the heat transfer in any gas. The heat transfer characteristics of air and SF6 at several pressures and in several practical geometries are then derived using these formulae.