Analysis for Variable Properties

The numerical-analysis procedure is now clear when the inlet temperatures and flows are given  

Calculate the value of U for the inlet conditions and through the initial AA increment. 

Repeat the foregoing steps until all the increments in A A are employed. The total heat-transfer rate is then calculated from 

A numerical analysis such as the one discussed above is best performed with a computer. Heat-transfer rates calculated from a variable-properties analysis can frequently differ by substantial amounts from a constant-properties analysis. The most difficult part of the analysis is, of course, a determination of the values of h. The interested reader is referred to the heat-transfer literature for additional information on this complicated but important subject. 

A rock-bed thermal-energy storage unit is employed to remove energy from a hot airstream and store for later use. The schematic for the device is shown in the sketch. The surface is covered with a material having an overall R value of 2 h °F ft2/Btu. The inlet flow area is 5 x 5 = 25 ft2, and the rock-bed length is 10 ft. Properties of the rock are 

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