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The heat transfer coefficient is often calculated from the Nusselt number (a dimensionless number). There are also online calculators available specifically for Heat-transfer fluid applications. Experimental assessment of the heat transfer coefficient poses some challenges especially when small fluxes are to be measured (e.g. < 0.2 W/cm 2). [1] [2]
Formulas and correlations are available in many references to calculate heat transfer coefficients for typical configurations and fluids. For laminar flows, the heat transfer coefficient is usually smaller than in turbulent flows because turbulent flows have strong mixing within the boundary layer on the heat transfer surface. [6]
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:
The third chart in each set was supplemented by Gröber in 1961, and this particular one shows the dimensionless heat transferred from the wall as a function of a dimensionless time variable. The vertical axis is a plot of Q/Q o, the ratio of actual heat transfer to the amount of total possible heat transfer before T = T ∞.
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:
The convective heat transfer between a uniformly heated wall and the working fluid is described by Newton's law of cooling: = where represents the heat flux, represents the proportionally constant called the heat transfer coefficient, represents the wall temperature and represents the fluid temperature.
The CLF is the cooling load at a given time compared to the heat gain from earlier in the day. [1] [5] The SC, or shading coefficient, is used widely in the evaluation of heat gain through glass and windows. [1] [5] Finally, the SCL, or solar cooling load factor, accounts for the variables associated with solar heat load.
The equation yields the surface averaged Nusselt number, which is used to determine the average convective heat transfer coefficient. Newton's law of cooling (in the form of heat loss per surface area being equal to heat transfer coefficient multiplied by temperature gradient) can then be invoked to determine the heat loss or gain from the ...