Thermal Design of an Airborne Computer Chassis With Air-Cooled, Cast Pin Fin Coldwalls

This paper documents the thermal design process required to provide effective thermal management for an airborne computer, consisting of 24 modules (two P/S modules and 22 PWB modules), which are edge-cooled to two cast, pin fin coldwalls. The computer chassis is mounted in an electronics pod mounted underneath the centerline of an aircraft. The pod consists of several electronics bays and a self-contained, air-cycle, environmental control system (ECS). The computer chassis is mounted in the forward bay, and the ECS is mounted in the rear bay of the electronics pod. The ECS is an air-cycle refrigeration system, which operates on captured ram air directed by an inlet/diffuser to an expansion turbine. This turbine produces low-pressure, chilled air, which is then directed through an air-to-liquid, load heat exchanger to produce chilled liquid. The chilled liquid is piped through small liquid lines to the forward bay of the pod, where the air-cooled computer chassis is located. The chilled liquid is converted back to chilled air in an air-to-liquid heat exchanger. The chilled air is supplied to the forward bay volume and is drawn through the computer chassis coldwalls by a fan integral to the computer chassis. The temperature of the chilled air, produced in this manner, becomes a strong function of the altitude and flight speed of the aircraft, because of the effect of these two parameters on the ram air mass flow rate and temperature at the inlet to the expansion turbine. The mass flow of the air used to cool the chassis is also a variable, because the density of the air is a function of the flight altitude and the fan has altitude-dependent operating characteristics. This fan provides the flow of air through the chassis. Emphasis is placed in the design process on the effect of the operating characteristics of the fan at altitude and the determination of the system performance curve associated with the pin fin coldwalls. This performance curve is controlled by the pressure drop characteristics of the pin fin coldwalls, which are a function of the Fanning f-factor and Colburn j-factor characteristics of the cast pin fin design. Design examples are used to demonstrate the design process.