NACA Report No. 106

NACA Report No. 106 - Turbulence in the Air Tubes of Radiators of Aircraft Engines was issued by the United States National Advisory Committee for Aeronautics in 1921.
Summary
NACA Report No. 106 describes an investigation of the flow characteristics in the air passages of aircraft radiators. This work was requested by NACA and was performed by the Bureau of Standards.
The primary requirement of a cooling radiator is that it shall dissipate heat; and for cooliug the engines of aircraft it is essential that the head resistance shalI be low. But both heat transfer and head resistance are greatly affected not only by the speed of air past the cooling surfaces, but by the character of the flow-whether the air passes through the radiator in smooth streams, or with eddies and vortices. If the flow is turbulent, the questions arise whether the turbulence can be increased by changes in construction, and if so whether the result is beneficial or harmful to the general performance of the radiator. This report presents experimental evidence bearing on the problem, and presents some conclusions based on that evidence.
Conclusions
Effects of turbulence upon radiator performance: in considering such effects, it is necessary to bear in mind the importance of the quantity of air flowing through the core, and the fact that at a given speed of flight rather widely different amounts of air flow through different radiators. The comparisons made in this report were based on a given airflow through the core, but speed of flight is the proper basis for comparing ihe general performance characteristics of a radiator. Any form of construction that imparts additional turbulence to the air may be expected to increase the resistance to flow of air and consequently to decrease the flow through the radiator for a given flying speed, while at the same time increasing the head resistance. If, then, there is to be a gain in general performimce, any device for producing turbulence must, by increasing the amount of cooling surface, or by causing the air to scour the surface more thoroughly, or both, increase the heat transfer enough to overbalance both the decrease in amount of air flow (which tends to decrease the heat transfer), and the increase in head resistance.
The general performance of four types of radiator, each representing one of the best of its class, is given in this report, and is awarded a 'figure of merit’, which is the ratio of the rate of dissipation of heat (expressed in units of power) to the power absorbed in overcoming the head resistance
and sustaining the weight of the racliator.
It is noticeable that at the higher speeds the flat plate and square cell types show much higher figure of merit than the other two types, although at a speed of 20 miles per hour the type with spiral vanes would perhaps be better than any of the others. The figures of merit as drawn apply only to radiators mounted in ‘unobstructed’ positions, such that the flow of air through and around them is practicably unaffected by other parts of the aircraft. For use in such positions at high speeds, every form of turbulence device known to this bureau is detrimental to the general performance of the radiator. On the other hand, if the radiator is to be used in such a position as the nose of the fuselage, the airflow through it at best is low, and an increase in airflow is accompanied by an increase in head resistance of the combination of fuselage and radiator. In this case, heat must be transmitted as rapidly as possible to the small amount of air that does flow through, and it may be profitable to use turbulence devices. It is possible that the rate of heat transfer for the whole radiator may be increased, while added air resistance of the core may actually reduce the head resistance of the fuselage and radiator.

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