In recent vehicle developments, increasingly sophisticated solutions for thermal management can be found, including electric water pumps and valves for individual segments of the cooling circuit. Thus, coolant flow to heat sources and heat exchangers can be provided on demand. For vehicles with start/stop functionality, heat transfer with stagnant coolant must be simulated in stop phases. In an increasing number of combustion engines, split cooling is implemented to reduce the warm-up period. With this technology, coolant flow to the engine block is stopped below a certain temperature level. Full transient flow control of coolant flows can also be found in hybrid and electric vehicles.
The new transient solver in KULI will improve the simulation support of recent vehicle thermal management developments. To demonstrate some aspects of the solver, a simple cooling circuit was set up (figure 1). The main features of the cooling circuit are controllable- flow and heat sources, as well as valve controlled flow to a radiator. On the cooling air side, the simulation model consists of two air inlets, area resistance, radiator, fan, built-in resistance and air outlet.
Figure 1: Cooling circuit to demonstrate KULI dynamic solver Features
To demonstrate transient effects, high gradients for coolant flow and heat supply changes were applied. Simulation was started at 20 °C for all components and with a heat supply of 50 kW (see figure 2). After a short time, the valve opens the coolant flow to the radiator. Once the target coolant temperature is reached, the volume flow is controlled to keep the temperature below the allowable maximum. After 70 seconds, the heat supply is switched off with the cooling air flow still on. Therefore, the coolant flow is stopped by the control valve and the radiator core is cooled down. At 150 seconds the heat supply is switched on again.
Figure 2: Diagram of heat supply, volume flow and temperatures
To support the interpretation of the transient temperature profile, four phases are shown in figure 3. In phase 1, the coolant in the radiator is fully warmed up (t <= 70s). When the heat supply is switched off, the heater core is cooled down by the air (phase 2; 70s < t < 150s). In the stagnant part of the coolant circuit, heat is exchanged by conduction. When the heat supply is switched on again (t = 150s), the cold coolant is swept out of the radiator (phase 3). After a short period of time the coolant temperature will reach the maximum value again. Due to the rapid changes of the heat supply, volume flow and temperature curves are not smooth. The dynamic solver will be an option additional to the current solver, and will be available with the next major release of KULI.
Figure 3: Four states of coolant temperatures in the radiator