Search results
Results from the WOW.Com Content Network
The rate of heat flow is the amount of heat that is transferred per unit of time in some material, usually measured in watts (joules per second). Heat is the flow of thermal energy driven by thermal non-equilibrium, so the term 'heat flow' is a redundancy (i.e. a pleonasm). Heat must not be confused with stored thermal energy, and moving a hot ...
The heat transfer coefficient has SI units in watts per square meter per kelvin (W/(m 2 K)). The overall heat transfer rate for combined modes is usually expressed in terms of an overall conductance or heat transfer coefficient, U. In that case, the heat transfer rate is: ˙ = where (in SI units):
describes heat transfer across a surface = Here, is the overall heat transfer coefficient, is the total heat transfer area, and is the minimum heat capacity rate. To better understand where this definition of NTU comes from, consider the following heat transfer energy balance, which is an extension of the energy balance above:
This equation shows that the temperature decreases exponentially over time, with the rate governed by the properties of the material and the heat transfer coefficient. [7] The heat transfer coefficient, h, is measured in , and represents the transfer of heat at an interface between two materials. This value is different at every interface and ...
Heat transfer rate, or heat flow per unit time, is denoted by ˙, but it is not a time derivative of a function of state (which can also be written with the dot notation) since heat is not a function of state. [5] Heat flux is defined as rate of heat transfer per unit cross-sectional area (watts per square metre).
The time rate of heat flow into a region V is given by a time-dependent quantity q t (V). We assume q has a density Q, so that () = (,) Heat flow is a time-dependent vector function H(x) characterized as follows: the time rate of heat flowing through an infinitesimal surface element with area dS and with unit normal vector n is () ().
The statement of Newton's law used in the heat transfer literature puts into mathematics the idea that the rate of heat loss of a body is proportional to the difference in temperatures between the body and its surroundings. For a temperature-independent heat transfer coefficient, the statement is:
Heat transfer is a discipline of thermal engineering that concerns the generation, use, conversion, and exchange of thermal energy between physical systems. Heat transfer is classified into various mechanisms, such as thermal conduction, thermal convection, thermal radiation, and transfer of energy by phase changes.