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Heat is a form of energy which transfers between bodies which are kept under thermal interactions. When a temperature difference occurs between two bodies or a body with its surroundings, heat transfer occurs. Heat transfer occurs in three modes. Three modes of heat transfer are described below. Conduction Convection and Radiation Conduction: In Conduction, heat transfer takes place due to a temperature difference in a body or between bodies in thermal contact, without mixing of mass. The rate of heat transfer through conduction is governed by the Fourier’s law of heat conduction. Q = -kA(dT/dx) Where, ‘Q’ is the heat flow rate by conduction ‘K’ is the thermal conductivity of body material ‘A’ is the cross-sectional area normal to direction of heat flow and ‘dT/dx’ is the temperature gradient of the section. Convection: In convection, heat is transferred to a moving fluid at the surface over which it flows by combined molecular diffusion and bulk fl...

Nusselt’s analysis of film condensation makes the following assumptions : The film of the liquid formed flows under the action of gravity. The condensate flow is laminar, and the fluid properties are constant. The liquid film is in good thermal contact with the cooling surface and therefore, the temperature at the inside of the film is taken equal to the surface temperature T s . Further, the temperature at the liquid-vapor interface is equal to the saturation temperature T sat  at the prevailing pressure. Viscous shear and gravitational forces are assumed to act on the fluid; thus normal viscous force and inertia forces are neglected. The shear stress at the liquid-vapor interface is negligible. This means there is no velocity gradient at the liquid-vapor interface. The heat transfer across the condensate layer is by pure conduction and temperature distribution is linear. The condensing vapor is entirely clean and free from gases, air, and non-condensing i...

The flow in the heat exchanger is considered under steady-state and fully insulated at the outer surface. Consider an infinitesimal part of the heat exchanging tube of area dA, where temperatures of hot and cold fluid are T h and T c , respectively. The figure below shows the temperature profile along the direction of flow in parallel flow and counter flow heat exchangers. Figure 1: Temperature profiles in parallel flow and counter flow heat exchangers The profile of temperature variation depends upon the heat capacity of the fluid. The profiles are different for both types of heat exchangers, therefore, expressions of LMTD are also different, derived as follows. heat exchange from infinitesimal area dA at any point of the heat exchanger is given by dQ = U.dA(T h – T c ) Where ‘U’ is the overall heat exchanger coefficient and ‘A’ is the heat transfer area of the heat exchanger. this heat is taken from hot fluid and given to cold fluid. the temperature of...

Lecture Series on Basic Thermodynamics by Prof.S.K. Som, Department of Mechanical Engineering, IIT Kharagpur. For more details on NPTEL visit http://nptel.ac.in

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