Convection

Experimental investigation was conducted to measure the convective heat transfer coefficient and thermal performance of plate fins and plate cubic pin-fins heat sinks, under natural convection regime. The investigation was conducted for Rayleigh number from 8×10 6 to 9.5×10 6 and input heat of 10 W to 120 W. The fin spacing and fin numbers are varied between 5-12 mm and 5-9, respectively. The results demonstrated that plate cubic pin-fins heat sinks have lower thermal resistance and higher heat transfer, compared to plate fins heat sinks. Heat transfer enhancement of new-designed heat sinks is about 10% to 41.6% higher, compared to normal pin-fins. Increasing fin spaces in all types of studied heat sinks cause lower thermal resistance. But, increasing fin numbers does not cause better heat transfer. The best heat sink design was a plate cubic pin-fin heat sink with 7 fins and 8.5 mm fin spacing. Finally, empirical equations have been developed to correlate the average Nusselt number as

The present paper deals with an experimental convective heat transfer investigation around a film-cooled, high-pressure gas turbine rotor blade mounted in a stationary, linear cascade arrangement. The measurements were performed in the von Karman Institute Isentropic Light Piston Compression Tube facility. The test blade was made of Macor glass ceramic and was instrumented with thin film gages. The coolant flow was ejected simultaneously through the leading edge (three rows of holes), the suction side (two rows of holes), and the pressure side (one row of holes). The effects of overall mass weight ratio, coolant to free-stream temperature ratio, and free-stream turbulence were successively investigated.

The mixed convection in an open trapezoidal lid-driven cavity connected with a channel is investigated in the present paper. Four different cases were considered depending on the movement of the cavity sidewalls. For case I, the left sidewall moves downward; for case II, the left sidewall moves downward and the right one moves upward; while for case III, only the right sidewall moves upward. A comparative case (case 0) is accounted when both sidewalls are assumed stationary. The base of the cavity is subjected to a localized heat source of constant temperature T h. The effects of Richardson number Ri and Reynolds number ratio Re r on the flow and thermal fields have been investigated. The results indicated that for cases I and II, the average Nusselt number increases with the increase of the Richardson number and Reynolds number ratio. Moreover, it was found that the maximum average Nusselt number occurs with case I. When the lid-driven speed is three times that of the inlet airflow velocity, the augmentations of the average Nusselt number compared with stationary walls are 163%, 158%, and 96% for cases I, II, and III, respectively.

ABSTRACT One of the most efficient ways to reduce the pollution and fuel consumption of an automotive engine is to downsize the engine, whilst maintaining a high level of power and torque. This is achieved by using turbochargers. In urban, and often in suburban, traffic conditions the engine power demand is weak in relation to the maximum power available, so the turbocharger runs at low speed. To appreciate and improve engine performance, it is necessary to know the characteristics of the turbomachinery in this functioning area, characteristics which are not given by turbocharger manufacturer. The reason for this lack of information will be explained and the experiments we are currently conducting at low turbocharger speed are presented. Experimentally, it has been demonstrated that the measured performances of the compressor are dependent on heat exchange (convection and conduction) and are also linked to the pressure and temperature of the lubricating oil. At the CNAM laboratory, the turbocharger test rig has been equipped with a special torquemeter, allowing rotation speeds of up to 120000 rpm, set up between the turbine and the compressor. The turbine is thus separated from the compressor and could be considered as a drive which provides mechanical power to the turbocharger (torquemeter + compressor + bearing unit). Temperature and pressure of the lubricating oil can be adjusted to an experiment’s requirements. The test bench lay out is described. To achieve accurate measurements and evaluate the influence of heat exchanges, tests have been carried out with the whole compressor thermally isolated and with preheated inlet air. The compressor can be assumed to be adiabatic, and the power given to the air flow can be calculated using the first law of thermodynamics. Mechanical bearing losses can be deduced from this calculation and torquemeter power, but also from measurements of lubricating oil flow, and oil temperature at inlet and outlet. The results of experiments for different lubricating oil temperatures and pressures and turbocharger speeds are presented. Real compressor characteristics curves are set up and a comparison of experimental mechanical power losses with a journal bearing CFD model is presented.

A numerical analysis is carried out to study the performance of mixed convection in a rectangular enclosure. Four different placement configurations of the inlet and outlet openings were considered. A constant flux heat source strip is flush-mounted on the vertical surface, modeling an integrated circuit chips affixed to a printed circuit board, and the fluid considered is air. The numerical scheme is based on the finite element method adapted to triangular non-uniform mesh elements by a nonlinear parametric solution algorithm. Results are obtained for a range of Richardson number from 0 to 10 at Pr = 0.71 and Re = 100 with constant physical properties. At the outlet of the computational domain a convective boundary condition (CBC) is used. The results indicate that the average Nusselt number and the dimensionless surface temperature on the heat source strongly depend on the positioning of the inlet and outlet. The basic nature of the resulting interaction between the forced external air stream and the buoyancy-driven flow by the heat source is explained by the heat transfer coefficient and the patterns of the streamlines, velocity vectors and isotherms.

: A sounding climatology of a variety of parameters commonly used to forecast deep, moist convection using upper-air observations is developed. The data set includes 0000 and 1200 UTC rawinsonde data (approximately 3629 soundings) from Laughlin AFB, TX from April September 1995 2004. Cloud-to-ground lightning data, surface observations, and severe weather reports from the Storm Prediction Center (SPC) SeverePlot2 Program were used to categorize soundings as representative of conditions for no convection, light convection, convection within vicinity, moderate severe convection, or severe convection. Indices, including convective available potential energy (CAPE) and mean layer CAPE (MLCAPE), along with sounding parameters and combinations of such as 0-2 and 0-6 km bulk shear, 700-500 mb lapse rate, lifted condensation level (LCL) and mean layer LCL (MLLCL) heights, are examined in an attempt to distinguish between moderate and severe convection. The results show that the 0-6 km bulk ...

Polydimethylsiloxane (PDMS) is a soft polymer that is primarily used for soft lithography (e.g., microfluidics and labon-chip devices) and also has wide range of applications, such as for thermomechanical actuators. The unique material properties of PDMS (such as the low values of Young's modulus) renders it to be an attractive material for applications where large range of deformations can be achieved with small variations in the actuating pressure (or actuating forces) thus providing good mechanical advantage. PDMS has been reported in the literature for microfabricating and testing thermally actuated microvalves (for microfluidics applications). These microvalves involve the thermal expansion of a fluid resulting in the deformation of a flexible PDMS membrane. Accurate numerical modeling of such thermo-mechanical actuators made from PDMS necessitates the knowledge of the temperature dependent mechanical properties of PDMS (such as Young's modulus) which is currently lacking in the literature. In this study large deformations were obtained for a thin flexible PDMS membrane (with a square footprint of 7.2 mm and thickness of 200 microns) that was microfabricated on the top of a hermetically sealed cavity (that was 3 mm deep) by subjecting the membrane to thermo-pneumatic pressure arising from the thermal expansion of air trapped in the hermetically sealed cavity and heated from below. This enabled the experimental determination of maximum displacement of the membrane as a function of actuating temperature and therefore the estimation of the temperaturedependent mechanical properties (e.g., Young's Modulus and Poisson's ratio) using parametric simulations using the finite element method (FEM) and based on linear elastic assumption for the deformation of PDMS. Using digital images of the convex shape of the deformed PDMS membrane the maximum deformation was measured as a function of temperature under steady state conditions. Computational Fluid Dynamics (CFD) based commercial solver (Ansys™ 2019R1®) was used to estimate the air pressure inside the hermetically sealed chamber as a function of temperature under steady state conditions (which was verified by analytical calculations). The values of pressure (obtained from CFD simulations) was used as the boundary condition in the FEM model (Ansys™ 2019R1®) for a fixed value of the Young's Modulus and Poisson's ratio to estimate the maximum deformation of the PDMS membrane. By parametric variation of the Young's Modulus and Poisson's ratio (for a particular operating temperature) the actual values were determined based on the computational result that matched the experimental data. The results show that the material

In seeking to understand the way in which heat is transferred from hotter to cooler regions of the Earth, it is important to have a general understanding of the physical composition of the Earth and, its various regions in which heat is transferred. This essay will provide a brief description of Earth's composition, and clarify what is entailed in each heat transfer process. It will seek to define and clarify their differences, and identify how each discussed process plays an important role in transferring heat from the hotter regions of the Earth to cooler regions.

An understanding of the mechanisms of heat transfer is becoming increasingly important in today’s world. Conduction and convection heat transfer phenomena are found throughout virtually all of the physical world and the industrial domain. A thermal insulator is a poor conductor of heat and has a low thermal conductivity. In this paper we studied that Insulation is used in buildings and in manufacturing processes to prevent heat loss or heat gain. Although its primary purpose is an economic one, it also provides more accurate control of process temperatures and protection of personnel. It prevents condensation on cold surfaces and the resulting corrosion. We also studied that critical radius of insulation is a radius at which the heat loss is maximum and above this radius the heat loss reduces with increase in radius. We also gave the concept of selection of economical insulation material and optimum thickness of insulation that give minimum total cost.

Combined free convection and forced convection from a flush-mounted uniform heat source on the bottom of a horizontal rectangular enclosure with side openings is studied numerically. The inlet opening allows an externally induced air stream at the ambient temperature to flow through the cavity and exits from another two openings placed top of the both
side walls. Two-dimensional forms of Navier-Stokes equations are solved by using control volume based finite element technique. Three typical values of the Reynolds numbers, based on the enclosure height, are chosen as Re = 50, 100 and 200, and steady, laminar results are obtained in the range of Richardson number as 0 ≤ Ri ≤ 10 and a fixed Prandtl number of 0.71. The parametric studies for a wide range of governing parameters show consistent performance of the present numerical approach to obtain as stream functions and temperature profiles. Heat transfer rates at the heated walls are presented in terms of average Nusselt numbers. The computational results indicate that the heat transfer coefficient is strongly affected by Reynolds number and Richardson number. An empirical correlation is developed by using Nusselt number, Reynolds number and Richardson number.

Nowadays, nanoparticles have been widely applied in liquids due to their great impact on the growth rate of heat transfer as well as the solutions for the problems raised from the use of particles larger than nano in various currents. Therefore, in this article, an attempt has been made to study the research carried out in the field of forced heat transfer of nanofluids inside channels and pipes. In this way, the authors are reviewing articles starting from 2018 for channels and 2017 for tubes/pipes. All the articles that are in the above category have been studied from different viewpoints. In this article, first, the type of flow and the relations used in this field are introduced and by comparing the published articles in terms of methodology, type and size of nanoparticles, volumetric ratios of nanoparticles, flow conditions, etc., so that the effect of these parameters can be studied on heat transfer. The following is a statistical study of articles in this field, which includes all articles published from the beginning to the present, which have dealt with this issue in theory, numerical and experimental. As a result, it becomes clear that the special geometric properties of channels and pipes, the insertion of obstacles in the flow path with special conditions and characteristics, as well as the boundary conditions defined in recent studies have considerable influence on the growth of heat transfer rate and the improvement of system efficiency.

This paper presents direct solar dryer for drying application likes as fruits, vegetables, and spices. Previous efforts on solar drying and recent developments of direct solar dryer for drying of cereal grains, fruits, vegetables and spices in the rural areas of the tropic and sub-tropic are critically examined in terms of drying performance and product quality and economic. The unpredictable rise and frequent scarcity of fossil fuel accelerated the continuous search for an alternative power source. Solar is one of the renewable and sustainable sources of power that attracted a large community of researches from all over the world. This is largely due to its abundant in both direct and indirect form. As such the development of efficient and inexpensive equipment for the drying of agricultural products using solar power evolved thereby improving the quality of the product as well as improving the quality of life. In this paper a review of direct solar is presented. It has been observed that use of direct solar dryer under natural convection and forced convection reduces moisture in very less time as well as drying rate is very fast than the sun drying.