My Cooling Improvement Project (hopefully)

Interesting subject. Four factors affect conduction of heat from one area to another are:

1) The difference of temperature (delta-T) between the warmer and colder areas;

2) The length of time (t) over which the transfer occurs;

3) The area (A) in common between the warmer and colder areas; and

4) The resistance (R) to heat flow and conduction (U) between the warmer and colder areas.

Heat flows from warmer to colder areas. The greater the delta-T, the greater the rate of heat transfer; the rate is directly proportional. The amount of heat (BTU) is directly proportional to the time span of heat transfer (t). BTU/H is the amount of heat transferred in one hour. The larger the area common to the warmer and colder areas through which heat flows, the greater the rate of heat conduction. [For the same material, the same length of time, the same delta-T, the amount of heat (BTU) transferred is directly proportional to the area (A).

The rate that heat flows through a material is a function of the material's resistance to heat flow (R) or conductivity (U). Conductivity (thermal transmittance) is used in calculations.

R=1/U; U=1/R

Conductivity of a material is determined by laboratory testing; it is the amount of heat (BTU) passing through 1 square foot of a material in 1 hour with a 1 degree F temperature differential on one side of the material compared to the other.

The steady state calculation of this would be:

U=BTU/sq.ft. hour degree F

We then determine the amount of heat transferred (Q):

Q = U x A x delta-T x t

Thermodynamic engineers are also concerned about the boundary layer of air against the material because air is an insulator. You will actually see a temperature gradient on both sides of the material in a steady state condition, but it reduces dramatically with air flow across the surface of the material.

Consider a car radiator. The calculations become more involved because the condition is no longer a steady state situation, but rather one with constantly changing variables, such as water temperature, air temperature, and the rate of water and air flow.

The engineer must also be concerned about the water pump and the fan. The fan must be able to move enough air across the radiator at engine idle, but also not restrict air flow at higher miles per hour speed. The concern at higher speeds is that the fan begins to act like an aircraft propeller on a dead engine; it acts almost like a solid disc, inducing tremendous drag. This has been addressed with flexible fan blades, clutch fans, and electric fans. Most aircraft propellers can be feathered, that is turning the blades into the wind; that has not yet been accomplished on cars.

Also consider the operating range of an engine. An engine produces a specific amount of waste heat at a 500 RPM idle over time, but produces a huge amount more at 5,000 RPM per the same time. It is a simple function of physics. The cooling system must be designed to operate efficiently from one end of the range to the other.

As to the question of the necessary speed of cooling water flow through an engine, the answer can surely be calculated by a thermodynamic engineer with the necessary parameters, such as combustion chamber heat, materials, surface areas, etc. Theoretically, over a given period of time, the higher the cylinder head temperature, the more coolant is required to flow. [Aircraft with radial engines have cowl flaps to regulate cylinder head temperatures.] Also, it is not so much the rate of coolant flow through the radiator in controlling the amount of heat exchange as is the temperature differential between the coolant and the cooling air at any given time, the greatest factor being the speed of air through the radiator. Remember, it is air that must carry away the heat from the radiator.

The ideal world would be a variable speed water pump combined with a variable speed fan along with a computer, thus keeping cylinder head temperatures within a specific design range for optimum efficiency.