G • COOLING BY PROTECTIVE FLUID FILMS 



values. The deviation appears to be more pronounced when the coolant 

 flows become larger. It is believed that the discrepancies between theories 

 and experimental data can be interpreted by the following discussions. 



Assessment of experimental errors. The errors inherent in the measure- 

 ments of the main stream gas temperature were probably the most serious. 

 It can be realized that the application of proper thermometric techniques 

 is difficult without disturbing the velocity profile of the gas stream. The 

 deterioration of thermocouples at high gas stream temperatures creates 

 another serious problem. Furthermore the radiation heat loss from the 

 thermocouple to the straightening-tube wall, the uncertainty as to the 

 recovery factor of the probe, the error in the location of the thermocouple 

 with respect to the temperature and velocity profiles, and the changes in 

 the calibration of the thermocouple due to chemical reaction between the 

 couple and the gas stream may contribute to error in the measurement of 

 the true gas temperature. 



Next, the error in the measurement of the wall temperature due to 

 the presence of the thermocouple tends to block off the flow of coolant 

 between itself and the hot wall of the specimen. Thus the thermocouple 

 indicates temperatures which are higher than those that would actually 

 exist on the surface of an undisturbed wall. 



Disposition of carbonaceous materials on the specimen surface may 

 cause the difficulty in obtaining reproducible experimental results. Since 

 carbon deposition decreases the permeability of the specimen it would 

 affect the coolant discharge pattern, the thermal conductivity of the 

 specimen surface, and therefore the measured wall temperature. There is 

 some radial heat loss from the back of the specimen to the holder which 

 was not measured. The effect of this loss in general is to decrease the 

 value of wall temperature for a given value of Q/W. 



Error arising from assumptions in the theory. As previously men- 

 tioned, the theory assumed that there is no wall temperature gradient in 

 the flow direction and hence no heat conducted along the wall. Actually, 

 there is an "inlet length" for the porous material where the temperature 

 distribution changes from that typical of flow in an uncooled pipe to the 

 final distribution for porous-wall cooling. This inlet length depends on 

 the conductivity of the porous material and is large for a material of high 

 conductivity. Hence the theoretical results show a better agreement with 

 measured values obtained with porous ceramic specimens than a porous 

 copper specimen. Furthermore the measured data indicates that the dis- 

 crepancy among various porous materials is reduced considerably when 

 a longer specimen is used. 



In the theory the coolant flow normal to the wall was assumed uni- 

 formly distributed over the surface of the specimen. It has been found in 

 experiments that the coolant leaves the surface in the form of a number 

 of isolated jets. Thus the transpiration-cooling process will be less efficient 



< 480 ) 



