Cooling calibration
The 2014 Formula One regulations present engineers with a host of challenges, not only in terms of designing all-new power units and energy recovery systems but in ensuring they are integrated as efficiently as possible into the car packages. One area of particular concern is the cooling systems, which must accommodate the heat rejection needs of both a turbo-supercharged internal combustion engine and two energy recovery systems.
By and large, modern racecars still rely on water-to-air coolers to remove heat from the engine, and oil-to-air coolers for the lubricating fluids, although oil cooling of electric motor/generators and batteries is also becoming more common. Invariably, these coolers are mounted in the sidepods of a car, and the primary concern when specifying these systems is ensuring that sufficient cooling capacity is available to keep the powertrain operating comfortably under race conditions.
Often teams will build radiator cores in-house to their own specification. The internal pipework and external fining of these units is optimised using CFD, to ensure the most efficient fluid flow through the system while providing the maximum possible surface area for cooling. In Formula One the cores need to be able to withstand in excess of 3.75 bar of pressure, the maximum allowed by the FIA regulations; at this pressure the boiling point of the coolant rises to around 120 C, allowing for smaller radiators to be run. This in turn allows for smaller inlets, reducing the drag penalty on the overall aero package, with the difference between the maximum and minimum cooling package accounting for up to 5% of the total downforce. However, there is a constant trade-off between aerodynamic drag and engine performance, with a 5 C increase in engine temperature reducing the power output by about 1 hp.
Before track testing and wind tunnel time in Formula One was limited, assessment of cooling systems could be completed either in wind tunnels or trackside. The reduction in time available to test cars though has seen overall aero package testing take centre stage, to the detriment of other areas of development (such as cooling optimisation) when it comes to assigning track or wind tunnel testing time. As one Formula One engineer has put it, “Generally we have very limited (or no) time within our ‘aerodynamic’ wind tunnels for coolant system development.” This has meant teams have had to outsource some of the testing for cooling components, using companies who provide thermal and aerodynamic testing of water, oil and charge coolers.
To test coolers, the units are usually mounted on either a suitable template or enclosed in a sidepod before being attached to a wind tunnel test section. One such commercially available facility is equipped with a 300 kW boiler to provide high-temperature coolant to the radiator cores. A boiler of such capacity is needed owing to the high levels of heat rejection in a modern race engine, with Formula One motors transferring anywhere up to 230 kW of heat into the coolant.
The key tools for assessing the cooling efficiency of a core are a number of temperature sensors placed at the inlet and outlet of the radiator, and pitot tubes to measure the inlet and outlet airflow. As temperature-controlled airflow is passed over the outer ‘finned’ surface of the radiator, a temperature-controlled flow of fluid is passed through the internal tube passages. The system is then run through a pre-determined test matrix of differing load and airspeed conditions, with the level of heat dissipation being derived for each condition.
If cores are tested in isolation, without being mounted in sections of the vehicle, a closed-loop wind tunnel can be used. The closed-loop system allows for the level of heat entering the airstream to be calculated and a heat balance obtained, in order to double-check the data gained from simulation.
Beyond the basic measurements of inlet and outlet coolant temperatures and airflow velocities at the cooler face, subcontractors have also started to use laser doppler anemometry (LDA) and thermal imagery to provide visualisation of core performance. The use of thermal imaging cameras gives a very clear idea of surface temperatures across the core, allowing for hot and cold spots or blockages to be identified quickly, while LDA allows for detailed analysis of air velocity over the cooler.
Given the likely complex nature of the cooling packages that will be present in 2014, facilities providing such services will no doubt be in greater demand than ever.
Written by Lawrence Butcher