Search results
Results From The WOW.Com Content Network
The heat transfer coefficient is the reciprocal of thermal insulance. This is used for building materials and for clothing insulation. There are numerous methods for calculating the heat transfer coefficient in different heat transfer modes, different fluids, flow regimes, and under different thermohydraulic conditions.
The heat transfer rate can be written using Newton's law of cooling as = (), where h is the heat transfer coefficient and A is the heat transfer surface area. Because heat transfer at the surface is by conduction, the same quantity can be expressed in terms of the thermal conductivity k:
However, the heat transfer coefficient is a function of the temperature difference in natural convective (buoyancy driven) heat transfer. In that case, Newton's law only approximates the result when the temperature difference is relatively small.
The heat transfer coefficient is also known as thermal admittance in the sense that the material may be seen as admitting heat to flow. [10] An additional term, thermal transmittance, quantifies the thermal conductance of a structure along with heat transfer due to convection and radiation.
Thermodynamic and mechanical heat transfer is calculated with the heat transfer coefficient, the proportionality between the heat flux and the thermodynamic driving force for the flow of heat. Heat flux is a quantitative, vectorial representation of heat flow through a surface. [3]
Q is the exchanged heat duty , U is the heat transfer coefficient (watts per kelvin per square meter), A is the exchange area. Note that estimating the heat transfer coefficient may be quite complicated. This holds both for cocurrent flow, where the streams enter from the same end, and for countercurrent flow, where they enter from different ends.
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 coefficient accounts for the time lag between the outdoor and indoor temperature peaks. Depending on the properties of the building envelope, a delay is present when observing the amount of heat being transferred inside from the outdoors. The CLF is the cooling load at a given time compared to the heat gain from earlier in the day. [1] [5]