Airflows inside passenger cars and implications for airborne disease transmission

Transmission of highly infectious respiratory diseases, including SARS-CoV-2 are facilitated by the transport of tiny droplets and aerosols (harboring viruses, bacteria, etc.) that are breathed out by individuals and can remain suspended in air for extended periods of time in confined environments. A passenger car cabin represents one such situation in which there exists an elevated risk of pathogen transmission. Here we present results from numerical simulations of the potential routes of airborne transmission within a model car geometry, for a variety of ventilation configurations representing different combinations of open and closed windows. We estimate relative concentrations and residence times of a non-interacting, passive scalar -- a proxy for infectious pathogenic particles -- that are advected and diffused by the turbulent airflows inside the cabin. Our findings reveal that creating an airflow pattern that travels across the cabin, entering and existing farthest from the occupants can potentially reduce the transmission.Comment: 8 pages, 6 figures + supplementa

Safety Study Related to Hydrogen Leakage from Fuel Cell Systems

The main challenge for the wide spread use of hydrogen in fuel cell systems is the safety concerns due to its ease of leaking, low-energy ignition, large flammability range, high buoyancy and diffusion rate in air. To alleviate concern of explosion during experiments, scientists are using helium as a stimulant for hydrogen safety studies. However, the equivalent behavior between the two gases only relies on numerical or experimental results, and the similarity is not connected by a theoretical correlation. This thesis assesses similarity relations using helium for hydrogen studies and develops a theoretical helium plume model. Meanwhile, a case study of leakage in fuel cell vehicles is simulated by Computational Fluid Dynamics (CFD). The accuracy of three different correlations, i.e., equal volumetric flow rate, equal buoyancy and equal concentration between helium and hydrogen was compared by CFD simulations validated by helium experiment in a 1/4 sub-scale residential garage model. The accuracy of these different methods at different leakage rate, stage of release, ventilation method and location was discussed. An updated theoretical helium plume model was validated by PIV (Particle Image Velocimetry) experiment and CFD. It is found that the new model could be used in estimating the plume size and velocity. In the case study of hydrogen leakage inside a FCV (Fuel Cell Vehicle), ventilation and sunroof show critical effect to reduce the level of hydrogen concentration accumulation

Thermal comfort models for indoor spaces and vehicles—Current capabilities and future perspectives

International audienceThroughout this paper, we reviewed the most popular thermal comfort models and methods of assessing thermal comfort in buildings and vehicular spaces. Most of them are limited to specific steady state, thermally homogenous environments and only a few of them address human responses to both non-uniform and transient conditions with a detailed thermo-regulation model. Some of them are defined by a series of international standards which stayed unchanged for more than a decade. The article proposes a global approach, starting from the physiological reaction of the body in thermal stress conditions and ending with the model implementation. The physiological bases of thermal comfort are presented, followed by the main thermal comfort models and standards and finishing with the current methods of assessing thermal comfort in practice. Within the last part we will focus mainly on thermal manikin experimental studies, and on CFD (computational fluid dynamics) numerical approach, as in our opinion these methods will be mostly considered for future development in this field of researc

Vertical ventilation concepts for future passenger Cars

We compared three vertical ventilation concepts to dashboard ventilation in a generic car cabin with the aim to improve thermal passenger comfort and energy efficiency of future cars. Temperatures were analysed with an infrared camera and local temperature sensors. Omnidirectional velocity probes were used to capture the fluid velocities and temperatures in the vicinity of thermal passenger dummies, which were used to simulate the thermal impact of the passengers. Further, the ventilation efficiency was measured with the tracer gas technique using humidity sensors in the vicinity of the dummies and in the air outlets. Besides the experimental investigations, the relevant flow cases were studied by Computational Fluid Dynamics simulations using the RANS method, providing insight into the complex and three-dimensional flow structures of the passenger compartment. Validation of the simulations with the experimental data revealed acceptable consistency, however, with local deviations indicating further need for experimental investigations. The ventilation efficiencies of the vertical ventilation concepts were at least comparable or even better as compared to dashboard ventilation. Regarding the comfort-relevant flow parameters, dashboard ventilation stood out with the lowest temperature stratification but revealed comfort-critical flow velocities. The vertical ventilation concepts allowed for comfortable velocities, but tended to produce comfort-critical temperature stratifications. Pursuing the equivalent temperatures, the vertical systems revealed an improved heating performance over dashboard ventilation. During summer and spring/fall conditions, low momentum ceiling ventilation as well as the combination of cabin displacement ventilation and low momentum ceiling ventilation were able to provide comfortable equivalent temperature distributions

Fire performance of residential shipping containers designed with a shaft wall system

seven story building made of shipping containers is planned to be built in Barcelona, Spain. This study mainly aimed to evaluate the fire performance of one of these residential shipping containers whose walls and ceiling will have a shaft wall system installed. The default assembly consisted of three fire resistant gypsum boards for vertical panels and a mineral wool layer within the framing system. This work aimed to assess if system variants (e.g. less gypsum boards, no mineral wool layer) could still be adequate considering fire resistance purposes. To determine if steel temperatures would attain a predetermined temperature of 300-350ºC (a temperature value above which mechanical properties of steel start to change significantly) the temperature evolution within the shaft wall system and the corrugated steel profile of the container was analysed under different fire conditions. Diamonds simulator (v. 2020; Buildsoft) was used to perform the heat transfer analysis from the inside surface of the container (where the fire source was present) and within the shaft wall and the corrugated profile. To do so gas temperatures near the walls and the ceiling were required, so these temperatures were obtained from two sources: (1) The standard fire curve ISO834; (2) CFD simulations performed using the Fire Dynamics Simulator (FDS). Post-flashover fire scenarios were modelled in FDS taking into account the type of fuel present in residential buildings according to international standards. The results obtained indicate that temperatures lower than 350ºC were attained on the ribbed steel sheet under all the tested heat exposure conditions. When changing the assembly by removing the mineral wool layer, fire resistance was found to still be adequate. Therefore, under the tested conditions, the structural response of the containers would comply with fire protection standards, even in the case where insulation was reduced.Postprint (published version

Experimental and transient computational fluid dynamic analysis of vehicle underhood in heat soak.

Simulation-based analyses of underhood compartments are proving to be crucially important in a vehicle development program, reducing test work and time-to-market. While Computational Fluid Dynamics (CFD) simulations of steady forced flows have demonstrated reliable, studies of transient natural convective flows in engine compartments under thermal soak are not yet carried out due the high computing demands and lack of validated work. The present work assesses the practical feasibility of applying the CFD tool at the initial stage of a vehicle development programme for investigating the thermally-driven flow in an engine bay. A typical vehicle underhood was reproduced in half-scale for laboratory investigations. Surface temperatures of components, airflow patterns induced by the buoyant forces as well as the spatial distribution of the air temperature were measured under both steady and transient thermal conditions. Temperature mappings were obtained with thermocouples whereas airflow magnitudes and directions were determined with Particle Image Velocimetry (PIV) instrumentation. The detailed measurements were used as reference for validating the corresponding CFD simulations carried out with the software VECTIS. Experimental and numerical data correlated well in steady state, both quantitatively and qualitatively. A computation procedure that enables pseudo time-marching simulations to be performed with significantly reduced CPU time usage, in comparison to traditional fully-conservative transient simulations, was also developed. The methodology used a unique combination of CFD solver parameters to overcome the computationally challenging problem of solving for momentum transport in time-marching mode and for a long period of physical time. The procedure was successful in providing a detailed and time-accurate flow and thermal simulation of the underhood model during transient cooling. Such simulation would not have been practically feasible with a standard transient simulation. A reduction in CPU processing time in excess of 90% was achieved with good correlation between the CFD predictions and the experimental data

Characterisation of the unsteady wake of a square-back road vehicle

Square-back shapes are popular in the automotive market for their high level of practicality. These geometries, however, are usually characterised by high aerodynamic drag and their wake flow dynamics present many aspects, such as the coexistence of long- and short-time unsteady modes, whose full comprehension is still far from being achieved. The present work aims to provide some contributions to this field. An extensive experimental campaign consisting of balance, pressure tapping, particle image velocimetry and single point velocity measurements has been carried out in order to characterise the dynamic behaviour of the wake developing downstream of a simplified square-back geometry. Tests have been performed considering the Windsor body, at a Reynolds number (based on the model height) of ReH = 7.7 × 10^5. New insights on how the long-time instability develops are provided. The instability is shown to stem from the mutual interactions between the four shear layers bounding the wake rather than being the result of the state of perturbation of a single shear layer. Changes in the level of interaction between two or more shear layers are also reported to affect the short-time unsteady modes. A drag reduction is reported every time the symmetry of the wake is restored, as a consequence of the increased amount of reverse flow impinging on the base of the model. This seems to be true regardless of the configuration considered (with or without wheels) and the type of optimisation strategy adopted, although it does not necessarily imply the complete suppression of the long-time instability. In fact, a certain level of mobility in the flow reversal seems to be inevitable every time the symmetry of the wake is restored. Several elements that can alter this behaviour are also identified. A change in the curvature of at least one of the four shear layers is shown to increase the frequency of the switches between bi-stable states, until eventually the long-time instability disappears replaced by low frequency flapping or swinging motions. Such changes can be triggered by applying perturbations on either a global scale or a more local scale. Overall, the results presented in this work help to bridge the gap between simplified geometries and more realistic automotive shapes, as far as the characterisation of the time averaged and main unsteady features of the wake is concerned, and provide insights that may allow in the future the design of more effective flow control systems for drag reduction