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Heat Transfer
Published in Robert K. McMordie, Mitchel C. Brown, Robert S. Stoughton, Solar Energy Fundamentals, 2021
Robert K. McMordie, Mitchel C. Brown, Robert S. Stoughton
Convection is thermal energy transfer between a flowing fluid and a surface. For example, when you cool from a fan blowing air across your skin, this is convection heat transfer. There are two major types of convective heat transfer: forced convection and free (or natural) convection. In forced convection, the fluid flow is forced by a pump, a fan, the wind etc. If the solid surface and the fluid are at different temperatures, there will be heat transfer between the surface and the fluid. Free convection occurs in the absence of a mechanism to force the fluid flow—the flow itself is caused by the temperature difference between the fluid and the solid surface. In order to understand free convection, consider a solid surface and an adjacent fluid that is at rest, and the surface and the fluid are at the same temperature. Since the fluid and surface are at the same temperature, there is no heat transfer between the surface and the adjacent fluid. Now increase the temperature of the surface so that there is a temperature difference between the surface and the fluid. This temperature difference will increase the temperature of the layer of fluid next to the solid surface. This fluid layer, being at a higher temperature than the bulk of the fluid further away from the solid surface, will also have a lower density than the bulk fluid away from the surface. This density difference will cause the fluid adjacent to the wall to rise due to buoyancy effects. For free convection, the surrounding fluid is essentially at rest with only the fluid adjacent to the solid surface circulating.
Convective Heat Transfer of Nanofluids in Porous Media
Published in Yasser Mahmoudi, Kamel Hooman, Kambiz Vafai, Convective Heat Transfer in Porous Media, 2019
Bernardo Buonomo, Davide Ercole, Yasser Mahmoudi, Oronzio Manca, Sergio Nardini
Convective heat transfer enhancement is always a current topic in thermal engineering applications related to the improvement of energy system performance, electronic cooling, nuclear reactors, solar energy systems, aerospace automotive, process industry, and so on. One of the techniques to realize the convective heat transfer augmentation is by means of solid materials, such as nanoparticles or porous media with higher thermal conductivity with respect to the traditional base fluids. The employment of such materials also allows a significant increase of heat transfer contact area inside the fluids, and the heat transfer passes from a surface exchange to a bulk or mass exchange. The coupling of nanoparticles and porous media with high thermal conductivity is an interesting and promising solution. Many research activities have been developed in the convective heat transfer of nanofluids in porous media as reviewed by Nield and Kuznetsov (2014), Kasaeian et al. (2017), Xu et al. (2019), and Khanafer and Vafai (2019). This chapter provides a review of the recent researches on natural, forced, and mixed convection as well as the governing equations related to the convective heat transfer. Some results in terms of comparison are provided in the section Comparisons among Different Configurations, and some remarks are pointed out in the Conclusions.
The Laws of Nuclear Heat Transfer
Published in Robert E. Masterson, Nuclear Reactor Thermal Hydraulics, 2019
Convective heat transfer is the transfer of heat to (or from) a fluid as a result of the macroscopic motion of the molecules of the fluid. In other words, in addition to a temperature difference, convection requires some mass transfer to occur. Convection is only possible if the motion of the fluid results in a net transfer of heat. A moving fluid with a temperature gradient will always transfer some heat from one part of the fluid to another. If the fluid is in motion, this heat transfer will involve some form of convection. Sometimes convection is further subdivided into diffusion and advection. Advection requires the bulk motion of a fluid, while diffusion does not. The diffusion of thermal energy is a molecular process that is similar to the conduction of heat through solids. However, when it applies to convection, the solid object is replaced by a liquid or a gas.
Thermal performance of nanofluids in elliptical zigzag tube: a numerical approach
Published in Particulate Science and Technology, 2023
Sumaia Bugumaa Abubaker Alammari, Muhammad Abbas Ahmad Zaini
As the findings are centered on the same fluid properties and pipe cross-section, so the effect solely relies on the pipe shape. Figure 6(a) shows the trend of the Nusselt number against the design of the tubes. Nusselt number is a non-dimensional parameter that provides a measure of the convective to conductive heat transfer ratio at the wall, whereby a good convective heat transfer is translated with a high magnitude of Nusselt number. Obviously, the Nusselt number is greater for the zigzag pipe than that of the straight pipe. This can be explained by a stronger flow resulting from the bending of the zigzag tube. Consequently, the heat transfer is more promoted with increasing Reynolds number due to the creation of high recirculation flow and a thin boundary layer (Zheng et al. 2013).
Modelling on temperature field of high-strength concrete pavement with the impact of internal curing
Published in International Journal of Pavement Engineering, 2022
Jiajia Zhang, Jun Zhang, Yuzhang Liu
Convective heat transfer is the transfer of heat from one place to another by the movement of fluids. Convection is the dominant form of heat transfer between pavement and fluid air. The rate of heat transfer between pavement and air (q in W/m2) due to convection is proportional to the overall temperature difference between the pavement surface and its surrounding air. It is normally expressed as follows: where Ta and T1 are the temperatures in Celsius of the fluid air and the surface of concrete pavement, respectively. The quantity β is called the convection heat transfer coefficient (W/(m2 °C)). The convective heat transfer coefficient is dependent upon the physical properties of the fluid and the physical situation. For convenient use, a constant value of β is used for temperature field calculation of concrete pavements constructed in different seasons, as displayed in Table 3. A model that can simulate temperature change of air in a day (24 h) was used in the calculation (Zhang et al. 2009b). It is expressed as
Drying of grape pomace with a double pass solar collector
Published in Drying Technology, 2019
Mustafa Aktaş, Seyfi Şevik, Ekin Can Dolgun, Burhan Demirci
In this equation is the mean velocity of air blown to the product whereas Ls represents the characteristic dimension equal to internal diameter of the circular drying chamber. During the experiments, velocity of drying air was measured between 0.70 and 0.90 m/s. Nusselt (Nu) number is the dimensionless parameter used in describing convective heat transfer and as a result of convection, it defines the enhancement of heat transfer through a fluid layer. Reynolds number plays an important role in Nusselt number. The Prandtl number (Pr) is a dimensionless number that is the ratio of the thermal diffusivity of the momentum diffuse. In laminar flow type, the following equation can be used to calculate Nusselt number.