This shows that the ‘surplus’ cooling capacity available for cooling the chamber and its contents, Hw, amounts to 221kW. This could be expected to achieve a rate of cooling of 221/30500=0.0072 deg C/s, or 1?C in 2.3min. However, this is the final rate when the temperature difference between walls and air had built up to the steady state value of about 20?C. The observed initial cooling rate is much faster, about 1.5?C/min. Heating presents less of a problem, since the heat released by the test vehicle and the fan assist the process rather than opposing it.
A further effect is associated with the moisture content of the tunnel air. On a warm summer’s day this could amount to about 10 kg of water and during the cooling process this moisture would be deposited, mostly on the cooler fins, where it would eventually freeze, blocking the passages and reducing heat transfer. To deal with this problem, it is necessary to include a dehumidifier to supply dried air to the tunnel circuit.