Heat and Mass Transfer HMT for Mechanical Engineering

The thermal conductivity of a material is a measure of its ability to conduct heat. It is commonly denoted by, or. Heat transfer occurs at a lower rate in materials of low thermal conductivity than in materials of high thermal conductivity.

In gases and liquids, conduction is due to the collisions and diffusion of molecules during their random motion.

Fourier's law

The law of heat conduction, also known as Fourier's law, states that the rate of heat transfer through a material is proportional to the negative gradient in the temperature and to the area, at right angles to that gradient, through which the heat flows.

Heat conduction in solids is similar to the conduction of electricity in electrical conductors in many aspects. In a conductor, the flow of electricity is driven by a potential difference and so is the flow of heat driven by a difference in temperature.

The problem of heat transfer through the composite system can be solved by the application of thermal resistance concept. In this lecture, We are considering a composite slab/wall having different thermal conductivity but same cross sectional area.

The critical radius of insulation is a counter-intuitive concept within the study of heat transfer. The critical insulation radius is defined as the thermal conductivity divided by the convection heat transfer coefficient; this ratio allows for maximum heat transfer, symbolically seen below.

r = k/h

In the study of heat transfer, fins are surfaces that extend from an object to increase the rate of heat transfer to or from the environment by increasing convection. The amount of conduction, convection, or radiation of an object determines the amount of heat it transfers.

If the temperature of a body does not vary with time, it is said to be in steady state. ... During this period, the temperature varies with time and body is said to be in the unsteady or transient state. This phenomenon is known as Unsteady or transient heat conduction.

Convective heat transfer, often referred to simply as convection, is the transfer of heat from one place to another by the movement of fluids. Convection is usually the dominant form of heat transfer in liquids and gases.

Forced convection is a mechanism, or type of transport in which fluid motion is generated by an external source. Alongside natural convection, thermal radiation and thermal conduction it is one of the methods of heat transfer and allows significant amounts of heat energy to be transported very efficiently.

The Reynolds number is the ratio of inertial forces to viscous forces within a fluid which is subjected to relative internal movement due to different fluid velocities. A region where these forces change behavior is known as a boundary layer, such as the bounding surface in the interior of a pipe.

The Nusselt number is the ratio of convective to conductive heat transfer across a boundary. The convection and conduction heat flows are parallel to each other and to the surface normal of the boundary surface, and are all perpendicular to the mean fluid flow in the simple case.

Prandtl number (Pr) is defined as the ratio of momentum diffusivity (kinematic viscosity) to thermal diffusivity.

Thermal Boundary Layer (TBL) is the layer of a liquid or gaseous heat-transfer agent between the free stream and a heat-exchange surface. In this layer the temperature of the heat-transfer agent changes from that of the wall to that of the free stream

Natural convection is a type of flow, of motion of a liquid such as water or a gas such as air, in which the fluid motion is not generated by any external source but by some parts of the fluid being heavier than other parts. The driving force for natural convection is gravity.

A heat exchanger is a system used to transfer heat between two or more fluids. Heat exchangers are used in both cooling and heating processes. The fluids may be separated by a solid wall to prevent mixing or they may be in direct contact.

The logarithmic mean temperature difference is used to determine the temperature driving force for heat transfer in flow systems, most notably in heat exchangers. The LMTD is a logarithmic average of the temperature difference between the hot and cold feeds at each end of the double pipe exchanger.

The logarithmic mean temperature difference is used to determine the temperature driving force for heat transfer in flow systems, most notably in heat exchangers. The LMTD is a logarithmic average of the temperature difference between the hot and cold feeds at each end of the double pipe exchanger.

Thermal radiation, process by which energy, in the form of electromagnetic radiation, is emitted by a heated surface in all directions and travels directly to its point of absorption at the speed of light; thermal radiation does not require an intervening medium to carry it.

Absorptivity (α) is a measure of how much of the radiation is absorbed by the body. Reflectivity (ρ) is a measure of how much is reflected, and transmissivity (τ) is a measure of how much passes through the object. Emissivity (ε) is a measure of how much thermal radiation a body emits to its environment.

In heat transfer, Kirchhoff's law of thermal radiation refers to wavelength-specific radioactive emission and absorption by a material body in thermodynamic equilibrium, including radioactive exchange equilibrium. A body at temperature T radiates electromagnetic energy.

So this here is the last lecture of the course. I hope you understood everything. The complete contents are attached herewith.

All the very BEST !!