The flow due to a disk rotating in its own plane has been investigated theoretically by von Kármán, Goldstein, and others, but little has been published on the heat transfer. For laminar conditions theoretical solutions have been given by Millsaps & Pohlhausen and by Wagner, but for conditions when the flow is turbulent over the outer part of the disk there is no previous information. The present paper describes an experimental investigation of the heat transfer for a range of conditions from entirely laminar flow to conditions when the outer 80% of the disk area is under turbulence. For laminar flow the heat transfer agrees with Wagner’s results, but Millsap’s theory is found to give too low values and an explanation is given. For the turbulent case, which occurs in most practical applications, values are given for the heat transfer which is found to approach the expression
N
= 0∙015
R
0∙8
for all-turbulent flow. An attempt is made to deduce the turbulent flow heat transfer theoretically by assuming a 1/7 power law of temperature distribution, but this gives too low a value. Some measurements of the velocity and temperature profiles both for laminar and for turbulent conditions are given. For laminar flow these show fair agreement with the theoretical values. For turbulent flow the temperature ratios are higher than those of velocity, which explains the low heat transfer values calculated assuming a 1/7 power temperature distribution. The relation between heat transfer and friction torque is also discussed.