Heat Transfer Dr. S.M. Mhatre

Heat transfer and its applications. Practically all the operations carried out by chemical involve the production or absorption of energy in the form of heat . Examples are ubiquitous: heat flows in the body home heating/cooling systems refrigerators, ovens, other appliances automobiles, power plants, the sun, etc. Dr. S.M. Mhatre

Heat A form of energy associated with the motion of atoms or molecules. Transferred from higher temperature objects to objects at a lower temperature. Dr. S.M. Mhatre

Temperature has only magnitude. It is a scalar quantity. Although heat transfer and temperature are closely related, they are of a different nature. Temperature has only magnitude. It is a scalar quantity. Heat transfer has direction as well as magnitude. It is a vector quantity. We work with a coordinate system and indicate direction with plus or minus signs. Dr. S.M. Mhatre

Work done by falling weights = mgh Calorie :The calorie had been defined as the amount of heat it takes to raise the temperature of 1 gram of water by 1 degree C. James Prescott Joule use his device below to find out how much work you would have to do to create a calorie of heat. Work done by falling weights = mgh The Mechanical Equivalent of Heat was found to be 4.2 Joules of mechanical work per calorie of heat produced 4.2 J/cal Dr. S.M. Mhatre

Task 1? A 10 kg cinder block is dropped 50 meters. How many calories of heat will it develop if dropped into 1000 kg water? SAT2: How much will the water’s temperature go up? Pool of water Pool of water Pool of water Dr. S.M. Mhatre

Heat Transfer There are 3 ways that heat can move from one place to another: radiation conduction convection Dr. S.M. Mhatre

Dr. S.M. Mhatre

Modes of Heat Transfer qconvection qradiation qconduction Conduction - diffusion of heat due to temperature gradient Convection - when heat is carried away by moving fluid Radiation - emission of energy by electromagnetic waves qconvection qradiation qconduction Dr. S.M. Mhatre

Typical Design Problems To determine: overall heat transfer coefficient - e.g., for a car radiator highest (or lowest) temperature in a system - e.g., in a gas turbine temperature distribution (related to thermal stress) - e.g., in the walls of a spacecraft temperature response in time dependent heating/cooling problems - e.g., how long does it take to cool down a case of soda? Dr. S.M. Mhatre

Task 2: Write three more design example related to heat transfer Dr. S.M. Mhatre

Conduction Heat Transfer Conduction is the transfer of heat by molecular interaction In a gas, molecular velocity depends on temperature hot, energetic molecules collide with neighbors, increasing their speed Dr. S.M. Mhatre

H = the Rate of Heat Flow through a conductor Unit: Joules/sec or Watts H = Q = k A T d Temperature difference Thermal Conductivity thickness Cross-sectional area Dr. S.M. Mhatre

Fourier’s Law “heat flux is proportional to temperature gradient” where k = thermal conductivity in general, k = k(x,y,z,T,…) units for q are W/m2 temperature profile 1 hot wall cold wall x Dr. S.M. Mhatre heat conduction in a slab

Thermal Properties Thermal Conductivity (k) It is the term used to indicate the amount of heat that will pass through a unit of area of a material at a temperature difference of one degree. The lower the “k” value, the better the insulation qualities of the material. Units; US: (Btu.in) / (h.ft2.oF) Metric: W / (m.oC) Conductance (c) It indicates the amount of heat that passes through a given thickness of material; Conductance= thermal conductivity / thickness Units; US: Btu / (h.ft2.oF) Metric: W/ (m2.oC) Dr. S.M. Mhatre

Thermal Resistance (RSI for metric unit, R for US units) It is that property of a material that resist the flow of heat through the material. It is the reciprocal of conductance; R= 1/c Thermal Transmittance (U) It is the amount of heat that passes through all the materials in a system. It is the reciprocal of the total resistance; U= 1/Rt Table 1 lists a few of the common materials and their thermal properties; Dr. S.M. Mhatre

Dr. S.M. Mhatre

Dr. S.M. Mhatre

Task 3: H = Q/T = k A T d A steel slab 5 cm thick is used as a firewall, measuring 3 m x 4 m. If a fire burns at 800 C on one side of a wall, how fast will heat flow through the metal door. (The conductivity of steel is 46 Watts/m•K) Dr. S.M. Mhatre