Heat Transfer free essay sample

Heat transfer Heat transfer is the transition of thermal energy from a hotter object to a cooler object (object in this sense designating a complex collection of particles which is capable of storing energy in many different ways). When an object or fluid is at a different temperature than its surroundings or another object, transfer of thermal energy, also known as heat transfer, or heat exchange, occurs in such a way that the body and the surroundings reach thermal equilibrium, this means that they are at the same temprature. If this ratio permits, it shows that the body has relatively high internal conductivity, such that (to good approximation) the entire body is at the same uniform temperature throughout, even as this temperature changes as it is cooled from the outside, by the environment. If this is the case, these conditions give the behavior of exponential decay with time, of temperature of a body. We will write a custom essay sample on Heat Transfer or any similar topic specifically for you Do Not WasteYour Time HIRE WRITER Only 13.90 / page In such cases, the entire body is treated as lumped capacitance heat reservoir, with total heat content which is proportional to simple total heat capacity C , and T, the temperature of the body, or Q = C T. Here, T(t) is the temperature at time t, and T(0) is the initial temperature at zero time, or t = 0. If: is defined as  : where is the initial temperature difference at time 0, then the Newtonian solution is written as: Uses: For example, simplified climate models may use Newtonian cooling instead of a full (and computationally expensive) radiation code to maintain atmospheric temperatures. One dimensional application, using thermal circuits A very useful concept used in heat transfer applications is the representation of thermal transfer by what is known as thermal circuits. A thermal circuit is the representation of the resistance to heat flow as though it were an electric resistor. The heat transferred is analogous to the current and the thermal resistance is analogous to the electric resistor. The value of the thermal resistance for the different modes of heat transfer are calculated as the denominators of the developed equations. The thermal resistances of the different modes of heat transfer are used in analyzing combined modes of heat transfer. The equations describing the three heat transfer modes and their thermal resistances, as discussed previously are summarized in the table below: In cases where there is heat transfer through different media (for example through a composite), the equivalent resistance is the sum of the resistances of the components that make up the composite. Likely, in cases where there are different heat transfer modes, the total resistance is the sum of the resistances of the different modes. Using the thermal circuit concept, the amount of heat transferred through any medium is the quotient of the temperature change and the total thermal resistance of the medium. As an example, consider a composite wall of cross- sectional area A. The composite is made of an L1 long cement plaster with a thermal coefficient k1 and L2 long paper faced fiber glass, with thermal coefficient k2. The left surface of the wall is at Ti and exposed to air with a convective coefficient of hi. The Right surface of the wall is at To and exposed to air with convective coefficient ho. Using the thermal resistance concept heat flow through the composite is as follows: Insulation and radiant barriers Thermal insulators are materials specifically designed to reduce the flow of heat by limiting conduction, convection, or both. Radiant barriers are materials which reflect radiation and therefore reduce the flow of heat from radiation sources. Good insulators are not necessarily good radiant barriers, and vice versa. Metal, for instance, is an excellent reflector and poor insulator. The effectiveness of an insulator is indicated by its R- (resistance) value. The R-value of a material is the inverse of the conduction coefficient (k) multiplied by the thickness (d) of the insulator. The units of resistance value are in SI units: (K ·m? /W)