This invention relates generally to the field of heat exchange systems, and in particular a thermal management system for cooling multiple devices operated in parallel and heated by high duty cycle long duration pulses.
Long duration pulsed devices require a coolant flow that is the same as the flow required for the continuous flow operation of the device even though the total heat load is reduced by the duty cycle of operation. Historically, for several devices operating in parallel, i.e., in a synchronized manner, (see FIG. 1), each is provided with a separate coolant flow, each of which is under-utilized.
FIG. 1 is a simplified diagram of four parallel synchronized devices with four parallel coolant loops. The flow is provided by a pump which circulates the flow through the devices and then the combined flow through the heat exchanger. The coolant flowing through the devices during the time between the heat pulses is not utilized for transporting heat to the heat exchanger. The heat exchanger performance depends upon the inlet temperature. The higher the inlet temperature the more effective is the heat exchanger. In this case the inlet temperature follows the heat pulsed profile so only operates periodically at peak efficiency. Between the heating pulses the coolant has a low temperature and thus the heat exchanger operates inefficiently. These periods of inefficiency are actually long compared to the effective time when the inlet temperature is high.
The flow required for continuous operation of the device at a power dissipation of P watts per device, is:
                    F        =                  P                      σ            ·            δ            ·                          Δ              ⁢              T                                                          (        1        )            
Where: F=coolant flow per device in liters/second                P=heat power per device, Watts        δ=density of the coolant, kg/liter        σ=heat capacity of the coolant, Joules/kg/C.°        ΔT=temperature increase in the coolant, C.°        
When the thermal pulse duration is longer than the thermal time constant of the device, typically 1 to 10 milliseconds, the required coolant flow reaches the continuous flow requirement given by equation 1. Control of the flow such that it is only on when the heat pulse is present would reduce the total flow requirement. However, starting and stopping the flow on a millisecond time scale is not practical or even possible to implement. The end result is a full continuous flow for each of the devices and a utilization of flow that is equal to the duty cycle of the heat pulses. The duty cycle is the ratio of the time the heat pulse is on to the repetition period of the heat pulses.