Heat Transfer Coefficient Of Air Research Articles

Performance improvement of wire mesh packed double-pass solar air heaters with external recycle

The device performance of the new design of wire mesh packed double-pass solar air heaters with attaching wire mesh under external recycle was investigated experimentally and theoretically. The improvement of device performance of wire mesh packed solar air heaters with different flow patterns is represented graphically and compared, including the single-pass, flat-plate double-pass with recycle and wire mesh packed double-pass with recycle. Heat transfer improvement is considerably obtained by employing such a recyclic double-pass with wire mesh packed, instead of using the flat-plate single-pass operation. The wire mesh packed double-pass device introduced in this study was proposed for aiming to strengthen the convective heat transfer coefficient for air flowing through the wire mesh packed bed, and to determine the optimal design on an economic consideration in terms of both heat transfer efficiency improvement and power consumption increment. The effect of recycle ratio on the heat transfer efficiency enhancement as well as the power consumption increment has been also delineated.

Internal heat transfer coefficients in microporous media with rarefaction effects

The internal heat transfer of different gases in microporous media was investigated experimentally and numerically. The experimental test section had a sintered bronze porous media with average particle diameters from 11 μm to 225 μm. The Knudsen numbers at the average inlet and outlet pressures of each test section varied from 0.0006 to 0.13 with porosities from 0.16 to 0.38. The particle-to-fluid heat transfer coefficients of air, CO2 and helium in the microporous media were determined experimentally. The results show that the Nusselt numbers for the internal heat transfer in the microporous media decrease with decreasing the particle diameter, dp, and increasing Knudsen number for the same Reynolds number. For Kn>0.01, the rarefaction affects the internal heat transfer in the microporous media. A Nusselt number correlation was developed that includes the influence of rarefaction. The computational fluid dynamics (CFD) numerical simulation was carried out to do the pore scale simulation of internal heat transfer in the microporous media considering the rarefaction effect. Pore scale three-dimensional numerical simulations were also used to predict the particle-to-fluid heat transfer coefficients. The numerical results without slip-flow and temperature jump effects for Kn<0.01 corresponded well with the experimental data. The numerical results with slip-flow and temperature jump effects for 0.01<Kn<0.13 are lower than the numerical results without rarefaction effects, but closer to the experimental data. The numerical results with rarefaction effects can accurately simulate the unsteady heat transfer in the microporous media.

Heat Transfer Characteristics of a Copper-nickel Multi Tube with Corrugated Copper Fins in a Cross Flow Heat Exchanger

A systematic experimental investigation was conducted to evaluate the heat Transfer characteristics of a copper-nickel multi tube with corrugated copper fins in a cross flow heat exchanger. The heat transfer rate from a tube can be augmented either by increasing the surface area, by number of fins or by creating turbulence in the flow. Experiment was carried out under the copper fin tube type heat exchanger in which no. of fins is mounted along the full length of heat exchanger. The effect of no. of fins on a full length heat exchanger on the heat transfer coefficient was experimentally investigated in case of different mass flow rate of water and air. The performance evaluation of cross flow heat exchanger with copper fins was carried out for water temperature varying from 38°C to 64°C and water velocity 0.07735m/s to 0.271m/s and air velocity 3.194m/s to 14.52m/s. It was found that on the basis of different flow rate of water and air heat transfer coefficient increases constantly on the water and air side respectively.

Heat transfer characteristics of a copper-nickel multi tube with corrugated copper fins in a cross flow heat exchanger

A systematic experimental investigation was conducted to evaluate the heat Transfer characteristics of a copper-nickel multi tube with corrugated copper fins in a cross flow heat exchanger. The heat transfer rate from a tube can be augmented either by increasing the surface area, by number of fins or by creating turbulence in the flow. Experiment was carried out under the copper fin tube type heat exchanger in which no. of fins is mounted along the full length of heat exchanger. The effect of no. of fins on a full length heat exchanger on the heat transfer coefficient was experimentally investigated in case of different mass flow rate of water and air. The performance evaluation of cross flow heat exchanger with copper fins was carried out for water temperature varying from 38°C to 64°C and water velocity 0.07735m/s to 0.271m/s and air velocity 3.194m/s to 14.52m/s. It was found that on the basis of different flow rate of water and air heat transfer coefficient increases constantly on the water and air side respectively.

Analytical and Experimental Studies on the Thermal Efficiency of the Double-Pass Solar Air Collector with Finned Absorber

Problem statement: The design of suitable air collectors is one of the most important factors controlling the economics of the solar drying. Air type collectors have two inherent disadvantages: Low thermal capacity of air and low absorber to air heat transfer coefficient. Different modifications are suggested and applied to improve the heat transfer coefficient between the absorber plate and air. These modifications include the use of extended heat transfer area, such as finned absorber. Approach: The efficiency of the solar collector has been examined by changing the solar radiation and the mass flow rate. An analytical and experimental study to investigate the effect of mass flow rate and solar radiation on thermal efficiency were conducted. The theoretical solution procedure of the energy equations uses a matrix inversion method and making some algebraic rearrangements. Results: The average error on calculating thermal efficiency was about 7%. The optimum efficiency, about 70% lies between the mass flow rates 0.07-0.08 kg sec-1. The thermal efficiencies increase with flow rate and it increase about 30% at mass flow rate of 0.04-0.08 kg sec-1. Conclusion: The efficiency is increased proportional to mass flow rate and solar radiation and .the efficiency of the collector is strongly dependent on the flow rate.

Methodology for the estimation of cylinder inner surface temperature in an air-cooled engine

A methodology for the estimation of the mean temperature of the cylinder inner surface in an air-cooled internal combustion engine is proposed. Knowledge of this temperature is necessary to determine the heat flux from the combustion chamber to the cylinder wall. Along with the heat transfer coefficient this parameter also allows almost 50% of engine heat losses to be determined. The temperature is relatively easy to determine for water-cooled engines but this is not in the case for air-cooled engines. The methodology described here combines numerical and experimental procedures. Simulations were based on FEM models and experiments were based on the use of thermocouples and infrared thermography. The methodology avoids the use of data or correlations developed for other engines, providing more reliable results than extrapolating models from one engine to another. It also prevents from taking measurements from inside the combustion chamber, reducing invasion and experiments complexity. The proposed methodology has been successfully applied to an air-cooled four-stroke direct-injection diesel engine and it allows the cylinder mean inner surface temperature and cylinder-cooling air heat transfer coefficient to be estimated.

A New Technique to Determine Convection Coefficients With Flow Through Particle Beds

Abstract A new method for determining the heat transfer coefficient for air flowing steadily through beds of particles is presented. In this technique, a step change in the inlet air temperature is applied to a small test bed and temperature distributions in the bed and at the air outlet are sampled over a short time period. The convective heat transfer coefficient is determined using data from the convective heat transfer process in the bed where the analysis includes the partial differential equation that describes the transient energy storage in the particles within the bed. The analysis is performed for a short time duration when the temperature distribution in the particle bed is almost linear along the axis of the bed. This time period permits the most accurate determination of the heat transfer coefficient using the data. Using beds of spherical particles a new correlation is developed for the Nusselt number versus the Reynolds number (5&amp;lt;Redh&amp;lt;280) and includes the uncertainty bounds. This new correlation compares well with correlations developed by some other researchers for similar spherical particle beds.

Heat Dissipation of Printed Circuit Board by the High Thermal Conductivity of Photo-Imageable Solder Resist

Thermal management issues with IC packages have been growing as electronic systems have become smaller with a higher functionality. Since the high junction temperature of IC packages induces the low performance and malfunction of electronic systems, the thermal dissipation capability of electronics is important for stable electrical performance and electro-mechanical reliability. However, the conventional cooling methods that depend on air flow path and heat sink structure is not sufficient to meet the growing thermal requirements of IC packages. Since there is limit on conventional design, such as optimization air flow path and heat sink structure, to dissipate more heat through an exhaust fan system. The main purpose of the present research is to reduce the junction temperature of the IC package by using a Printed Circuit Board (PCB) coated with new Photo-imageable Solder Resist (PSR) that has high thermal conductivity. The thermal conductivity of the newly developed PSR is about five times as high as that of the conventional PSR for PCB(0.23∼0.25W/m·K). The PCB was prepared by the EIA/JEDEC standard, JESD51-7. Experimental and finite element analyses were performed to investigate the effect of SR on thermal dissipation capability. The experimental and FE results show that the high thermal conductivity of PSR can reduce the steady-status regulator surface temperature by about 3∼8K as an air flow conditions around PCB. Also, the high thermal conductivity of SR is more effective under a low air heat transfer coefficient condition. Therefore, it is believed that an improved heat transfer with a PSR of high thermal conductivity should provide stable electrical performance for the IC package.

Raman Measurements of Thermal Transport in Suspended Monolayer Graphene of Variable Sizes in Vacuum and Gaseous Environments

Using micro-Raman spectroscopy, the thermal conductivity of a graphene monolayer grown by chemical vapor deposition and suspended over holes with different diameters ranging from 2.9 to 9.7 μm was measured in vacuum, thereby eliminating errors caused by heat loss to the surrounding gas. The obtained thermal conductivity values of the suspended graphene range from (2.6 ± 0.9) to (3.1 ± 1.0) × 10(3) Wm(-1)K(-1) near 350 K without showing the sample size dependence predicted for suspended, clean, and flat graphene crystal. The lack of sample size dependence is attributed to the relatively large measurement uncertainty as well as grain boundaries, wrinkles, defects, or polymeric residue that are possibly present in the measured samples. Moreover, from Raman measurements performed in air and CO(2) gas environments near atmospheric pressure, the heat transfer coefficient for air and CO(2) was determined and found to be (2.9 +5.1/-2.9) and (1.5 +4.2/-1.5) × 10(4) Wm(-2)K(-1), respectively, when the graphene temperature was heated by the Raman laser to about 510 K.

Experimental determination of natural convection heat transfer coefficients in an attic shaped enclosure

Heat transfer by natural convection in triangular enclosures is an area of significant importance in applications such as the design of greenhouses, attics and solar water heaters. However, given its significance to these areas it has not been widely examined. In this study, the natural convection heat transfer coefficients for air in an attic shaped enclosure were determined for Grashof Numbers over the range of 10 7 to 10 9. It was found that the measured heat transfer coefficients could be predicted to within 5% by Ridouane and Campo's [E.H. Ridouane, A. Campo, Experimental-based correlations for the characterization of free convection of air inside isosceles triangular cavities with variable apex angles, Experimental Heat Transfer 18 (2) (2005) 81–86] equation (Eq. (1)) for natural convection in a triangular enclosure previously developed for Grashof Numbers in the range of 10 5 to 10 6. (1) N u = 0.286 A − 0.286 G r 1 / 4 . As such, it is suggested that this equation may be suitable for predicting the natural convection heat transfer coefficients in full scale attic enclosures.

Analysis of Strip Temperature in Hot Rolling Process by Finite Element Method

The program was developed by finite element method to calculate the temperature distribution in hot strip rolling process. The heat transfer coefficients of air cooling, water cooling and thermal resistance between work roll and strip were analyzed. A new heat generation rate model was proposed according to the influence of source current density, work frequency, air gap and distance to edge on induction heating by finite element method (FEM). The heat generation rate was considered in the thermal analysis to predict the temperature distribution in the induction heating. The influence of induction heating on the strip temperature was investigated for different slab thicknesses. The temperature difference became more and more obvious with the increase of thickness. The strip could be heated quickly by the induction heating both in surface and center because of the thermal conductivity and skin effect. The heat loss of radiation has important influence on the surface temperature. The surface temperature could be heated quickly by high frequency when the strip is thicker.

MULTIPHASE SPRAY COOLING OF STEEL PLATES NEAR THE LEIDENFROST TEMPERATURE — EXPERIMENTAL STUDIES AND NUMERICAL MODELING

Experimental studies were conducted toreveal the heat transfermechanism of impacting water mist on high-temperature surfaces. A numerical model was developed to simulate, for atmospheric applications, air and water mist spray cooling of surfaces heated to temperatures ranging from nucleate to film boiling. Local heat transfer coefficients were measured in the film-boiling regime at various air velocities and liquid mass fluxes. The test conditions of water mist cover the variations of air velocity from 0 to 50.3 m/s, liquid mass flux from 0 to 7.67 kg/m 2 s, and surface temperature of stainless steel between 525 � C and 500 � C. Radial heat transfer distributions were measured at different liquid mass fluxes. The tests revealed that the radial variation of heat transfer coefficients of the water mist has a similar trend to that of air jet cooling. At the stagnation point, the heat transfer coefficient increases with both the air velocity and the liquid mass flux. The convective air heat transfer is consistent with the published correlation in the literature. The heat transfer contribution due to the presence of water increases almost linearly with the liquid mass flux. For dilute sprays, the total heat transfer coefficient can be established as two separable effects, which is the summation of the heat transfer coefficient of air and of liquid mass flux. This study shows that with a small amount of water added in the impacting air jet, the heat transfer is dramatically increased. The Leidenfrost temperature associated with the water mist cooling was also measured. The Leidenfrost temperature increased with both the air velocity and the liquid mass flux. The model simulation was compared against available test data at atmospheric conditions, and the simulation compared favorably well with the test data.

Experimental study on thermal characteristics of suspended platinum nanofilm sensors

In this paper, the thermal characteristics of suspended platinum (Pt) nanofilm sensors have been investigated experimentally. The Pt nanofilm sensors with the thickness of 28–40 nm, the width of 260–601 nm, and the length of 5.3–5.7 μm were fabricated by electron beam lithography, electron beam physical vapor deposition and isotropic/anisotropic etching processes. Based on the one-dimensional heat conduction model, the in-plane thermal conductivity of the nanofilm sensors was obtained from the linear relation of the volume-averaged temperature increase and the heating rate measured in vacuum. Furthermore, natural convection heat transfer coefficients of air around the suspended nanofilm sensors at the pressures ranging from 1 × 10 −2 Pa to 1 atm were also investigated. The experimental results show that the in-plane thermal conductivities of the nanofilm sensors are much lower than those of the bulk values, the natural convection heat transfer coefficients are, however, very high at the atmospheric pressure.

A new technique for dynamic heat transfer measurements and flow visualization using liquid crystal thermography

A new technique for dynamic heat transfer coefficient measurements and flow visualization is described. This technique uses a surface with a low thermal mass (a thin tissue) embedded with thermochromic liquid crystals (TLCs) and heated uniformly with infrared radiation. For air heat transfer coefficient measurements, the frequency response is estimated at 0.3–0.5 Hz. Depending on the local heat transfer coefficient and the magnitude of its fluctuations, it is estimated that surface temperature fluctuations can be detected to 100 Hz. These surface temperature fluctuations are driven by changes in local heat transfer coefficients, caused by dynamic flow behavior such as vortex shedding, and are captured by video recordings of the hue of the liquid crystals. The video images provide time-dependent heat transfer coefficient distribution and time-dependent surface flow visualization. Two applications are used to illustrate this technique: flow on a surface downstream of a protruding cylinder in cross-flow with vortex shedding, and flow downstream of a shallow cylindrical surface dimple. Images of the time-dependent surface temperature distribution downstream of the protruding cylinder are presented. They show the fluctuations in the surface heat transfer coefficient due to vortex shedding. The temperature distributions downstream of the cylindrical dimple were found to be relatively steady within the frequency limits of this technique. Heat transfer coefficient contours are presented for this case.

Heat Transfer of Impacting Water Mist on High Temperature Metal Surfaces

Experimental studies were conducted to reveal the heat transfer mechanism of impacting water mist on high temperature metal surfaces. Local heat transfer coefficients were measured in the film-boiling regime at various air velocities and liquid mass fluxes. The test conditions of water mist cover the variations of air velocity from 0 to 50.3 m/s, liquid mass flux from 0 to 7.67 kg/m2s, and surface temperature of stainless steel between 525°C and 500°C. Radial heat transfer distributions were measured at different liquid mass fluxes. The tests revealed that the radial variation of heat transfer coefficients of water mist has a similar trend to the air jet cooling. At the stagnation point, heat transfer coefficient increases with both the air velocity and the liquid mass flux. The convective air heat transfer is consistent with the published correlation in the literature. The heat transfer contribution due to the presence of water increases almost linearly with the liquid mass flux. The total heat transfer coefficient can be established as two separable effects, which is the summation of the heat transfer coefficient of air and of liquid mass flux, respectively. This study shows that with a small amount of water added in the impacting air jet, the heat transfer is dramatically increased. The Leidenfrost temperature under water mist cooling was also measured. The Leidenfrost temperature increased with both the air velocity and the liquid mass flux.

Experimental investigation of a cooled thick fin in dry and humid air flows

The results of a study are reported in which a cooled, thick vertical fin was tested in a closed loop tunnel with and without condensation from the air flowing over it. In particular, the temperature distributions for the dry and wet fin cases, together with the condensate film thickness in the wet fin case, were investigated. From a flow visualization investigation, it was found that the boundary layer separates at the leading edge, resulting in a higher air heat transfer coefficient. The wet fin test results also indicated that the mode of condensation was dependent on fin surface characteristics and that the wet fin performance was governed by the air flow parameter. Within the laminar air flow range, the condensate film flowed downward under the action of gravity. However, at higher air velocities, both gravity and shear forces affected the condensate flow, a variation in the condensate film in the direction of air flow being noticed.

Heat transfer in turbulent decaying swirl flow in a circular pipe

Heat transfer coefficients for air are measured along a heated pipe for decaying swirl flow, generated by radial blade cascade. The results are compared with an expression proposed for predicting the heat transfer coefficients in swirling flow. The theoretical predictions are in good agreement with the experimental data with average and maximum deviations of 7 and 11%, respectively. The application of the theoretical approach to the experimental results obtained by other investigators for heat transfer in a decaying swirl flow generated by short-twisted tapes and tangential slots at inlet also give to encouranging agreement.

Heat and mass transfer from a horizontal tube of an evaporative heat dissipator

In this investigation, the mass transfer coefficient with simultaneous water and air flow has been determined experimentally. The coefficient has also been calculated theoretically from the convective heat transfer coefficient of air by applying the Lewis relation for an air-water mixture. The ratio of experimental to theoretical mass transfer coefficients has been found to lie between 0.80 to 9.35. A new term ‘evaporative effectiveness’' is defined as the ratio of energy dissipated in evaporative cooling to that in simple water cooling. Its variation is studied and found to range from 0.85 to 1.78. Correlations for design purposes are recommended in this paper.

Polar bear thermoregulation: Effect of oil on the insulative properties of fur

1. 1.|Oil caused a substantial decrease in the insulative value of polar bear ( Ursus maritimus) pelts measured in vitro. 2. 2.|Following oil contamination the calm air heat transfer coefficient increased by a factor of 2 to 5: the wind coefficient averaged 290% greater and the solar utilization increased by 55%. 3. 3.|Conductance through oil-covered furs remained high at winter temperatures ( T a = 0.6° C) but decreased with time at summer temperatures ( T a = 24.7° C). 4. 4.|The most viscous of the three oils tested had a more consistently negative effect on insulation.

Heat Transfer Coefficients for Finned Exchangers

The effect of the number of staggered rows on the mean outside heat transfer coefficient for air flowing across extended surface tubes (finned tubes) has been determined experimentally. When the heat transfer coefficient is reported as a function of the Reynolds number, both the power to which the Reynolds number is raised and the constant of proportionality vary with the number of rows. A general correlation has been developed relating the outside heat transfer coefficients the Reynolds number, and the number of rows. The maximum deviation from this equation is 35%, with an average deviation of plus or minus 4.5%. Curves, tables, and equations are given for design purposes.