Efficient and Comfortable Cabin Ventilation
According to the current White Paper on “Transport” published by the European Union, the objective to reduce greenhouse gas emissions in the transport sector by 60% until 2050 (compared to 1990 levels) has been defined. To achieve this goal, e-mobility must be promoted, the attractiveness of rail transport must be increased and the efficiency of all sub-components must be improved.
In addition to traction, air-conditioning systems represent one of the main consumers requiring up to 50% of the total energy in case of road vehicles and 20-30% in case of rail vehicles. At the same time, the increasing degree of automation and the growing number of "people movers" create greater freedom in terms of the interior design, which must be supported by new ventilation concepts. Therefore, the aim of the studies carried out in the department Ground Vehicles is to develop ventilation concepts offering a high degree of efficiency while also ensuring thermal comfort without compromising traffic safety. The great synergy potential is optimally used by means of a knowledge transfer regarding the cabins of different transport modes: road and rail vehicles (Generic Train Laboratory Göttingen) as well as single-aisle (DO 728) and twin-aisle aircraft (Advent project).
In order to simulate the heat emission of passengers experimentally, more than 150 thermal manikins are available in the department. These manikins provide a comparable obstruction as seated passengers and can emit the same amount of sensible heat in the range of 0 - 150 W by means of an electric heating wire.
The numerical and experimental work focuses on vertical ventilation concepts, which – in addition to efficiency advantages compared to high-momentum mixed ventilation concepts – also allow a highly flexible interior design. This is particularly important for autonomous driving, but also for new innovative concepts for rail vehicles and aircraft cabins. New technologies such as component-integrated infrared radiation elements are also used when designing new concepts, especially with regard to personalised comfort zones.
In addition to investigating the entire vehicle cabin, individual components such as cabin air outlets, are analysed and optimised in experimental and numerical test campaigns.
Thermal Comfort and Air Quality
The thermal comfort of the passenger is an essential design criterion for the HVAC system and the other features of a vehicle and aircraft cabin. In the department Ground Vehicles, cabins of road vehicles, rail vehicles and aircraft (single- and twin-aisle) are investigated both numerically and experimentally.
Besides local air and radiation temperatures, air velocities, humidity and CO2 concentration, velocity fields are also measured over several square meters using large-scale Particle Image Velocimetry. For this purpose, buoyancy-neutral helium-filled soap bubbles are used as tracer particles illuminated by a laser or LEDs and analysed using correlation algorithms. Furthermore, thermal manikins have been and are being further developed in such a way that — after precise calibration — they can be used to determine the time-resolved, body segment related thermal comfort within the framework of equivalent temperature measurements. In addition to this enhancement of thermal manikins from pure heat sources for the experimental simulation of passengers to versatile measuring devices, an automatic adaptation of the heat emission comparable to the human metabolism is being investigated.
Computational Fluid Dynamics (CFD) simulation is another important tool for investigating the efficiency and comfort with regard to the air-conditioning system of a vehicle. The velocities and temperature distribution inside the vehicle cabin are calculated using numerical models. To determine the thermal comfort, the simulations are combined with the THESEUS-FE comfort model. This provides important local comfort parameters (ZHANG indices, PPD, PMV, equivalent temperature, ...), which deliver information about the thermal comfort of the driver and the other passengers.
The studies on thermal comfort in the department Ground Vehicles are supplemented by human subject tests carried out in cooperation with the Institute of Aerospace Medicine. The test persons have to complete questionnaires regarding different ventilation concepts and temperature settings both in the Do728 and in the generic train laboratory. Thus, the numerical and experimental comfort predictions can be validated by means of subjective ratings.
The process of condensation and resublimation as well as evaporation, melting and sublimation on an overflowed surface in convective air flows is a phenomenon we encounter in our everyday life, in nature and especially in various technical applications. Misted windscreens, for example, are an effect we are all familiar with. Defogging and defrosting of vehicle windscreens is of particular interest, as it is on the one hand relevant to road safety and on the other hand has negative effects from an energy point of view. Since the thermal energy is no longer inherently available as engine waste heat in vehicles powered by electric motors, defrosting the vehicle windscreen results in a significant loss of range, as a considerable amount of energy must be drawn from the battery. It is important to keep in mind that the phase transition of evaporation requires almost one magnitude of energy more compared to the energy required to heat water from 0°C to 100°C.
Furthermore, condensation in convective channel flows is a problem not to be underestimated. For example, condensation water in air-conditioning or ventilation systems can promote mould growth and thus have a negative effect on health. Condensation is also a challenge in aviation. During the cruise phase, condensation and resublimation occur in the frame between the aircraft’s fuselage and the primary insulation. During the ground phase, water and ice melt and evaporate there. This produces a considerable amount of water, which on the one hand causes damage through corrosion and on the other hand penetrates the insulation packs, where it eventually accumulates. The accumulation of water in the packs in turn reduces the thermal and acoustic insulation. In addition, the increased weight, which can be up to several hundred kilograms, leads to increased fuel consumption and thus also to a less favourable CO2 balance.
In convective air flows with phase transition, the mass and heat transfer are determined by the complex interaction of latent and sensible heat, diffusion and the surface properties of the interface where the phase transition occurs. To be able to analyse these processes, a fundamental understanding of the physical processes is necessary. For this purpose, experimental and numerical investigations are carried out in the department Ground Vehicles within the framework of basic research projects and doctoral/master/bachelor theses. Based on these results, we are able to transfer the complex topic of phase transition in convective air flows to application-related problems and configurations with a close connection to reality.