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processes of heat transfer to and from the fluid in the cooler, heater, and regenerator. Density and viscosity are important in relation to the How friction losses, which control the pump work necessary to move the lluid about the engine to perform the desired heat transfers from the heater or to the cooler. These How losses are directly proportional to pirf2 where p is the gas density and u the gas velocity.

The density of a gas may be calculated from the characteristic gas equation as:

where p is the pressure

M is the molecular weight R is the universal gas constant and I is the absolute temperature. Therefore, for a given pressure and temperature the density /> is directly proportional to the molecular weight, M.

The heat transfer processes occurring may be characterized by the equation:

where Q =heat transferred h = heat transfer coefficient .A = area for heat transfer

AT ~ temperature difference between the lluid and solid wall. The heat transfer coefficient h is one component of a dimensionless group called the Nussclt number:

/i = heat transfer coefficient k = thermal conductivity c = heal capacity. Another important dimensionless group of parameters involved in con-vective heat transfei processes is the Reynolds number:

where Re = Reynolds number p = density u =gas velocity d =a characteristic dimension of the flow

and 13 and q are constants which depend on the flow conditions. Therefore:

The best working fluid is the gas which combines a high heat transfer coefficient (large /ยก) with low friction or pumping losses (low pir).

fn general, hydrogen has the best combination of transport properties. It will result in less friction losses than helium or air for a given heat transfer rate at a given pressure or temperature situation. Alternatively, for a given flow loss in an engine, at a particular pressure and temperature level, the engine can run taster with hydrogen than with helium or air so that it has a higher specific output.

Tlie flow situation in a Stirling engine is so complex that it is not easy to compare numerically the advantages of one working fluid and another without becoming involved in the intricacies of advanced computer simulation studies. A better approach is to consider a steady-flow situation where an analogous combination of good heat transfer and low pumping losses is important. This exists in many engineering situations but is particularly important in the gas-cooled nuclear reactor. Ilall (1958) has given an excellent model for reactor heat transfer and the following treatment is an abbreviated version of his comparison of coolants.

The Reynolds analogy between fluid friction and heat transfer may be expressed in the form:

where f = Fanning friction factor St = Stanton number h = heat transfer coefficient p = density of fluid u = fluid velocity c =heat capacity of fluid.

The pressure drop with fluid flowing in a channel:

Steady-flow analysis

## Solar Stirling Engine Basics Explained

The solar Stirling engine is progressively becoming a viable alternative to solar panels for its higher efficiency. Stirling engines might be the best way to harvest the power provided by the sun. This is an easy-to-understand explanation of how Stirling engines work, the different types, and why they are more efficient than steam engines.

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