Vehicle Side-Window Defrosting and Demisting Process

Transcription

1 Vehicle Side-Window Defrosting and Demisting Process A. Aroussi, A. Hassan Flow Diagnostics Laboratory, School of M3EM, The University of Nottingham, UK. ABSTRACT The thermal comfort of passengers within a vehicle is often the main objective for the climate control engineer; however, the need to maintain adequate visibility through the front and side windows of a vehicle is a critical aspect of safe driving. This paper compares the performance of the side window defrosting and demisting mechanism of several current model vehicles. The study highlights the drawbacks of current designs and points the way to improved passive defrosting mechanisms. The investigation is experimental and computational. The experiments are carried out using full-scale current vehicle models. The computational study, which is validated by the experiments, is used to perform parametric investigation into the side window defroster s performance. The results show that current designs of the side-defroster nozzles give maximum airflow rates in the vicinity of the lower part of the window, which yields unsatisfactory visibility. Keywords: Side window defrosting, CFD validation, thermal imaging, laser sheet visualization, Ford Focus, Ford Taurus, AutoCAD 1. INTRODUCTION Good visibility through the side windows is a critical requirement for safe driving. This is more so in cold climates where both ice and mist may form on the glass surfaces. The side window area or target area required for the viewing of the side mirrors is vehicle specific. It depends on the relative position of the side mirrors with respect to the driver. The area to be cleared on the window closer to the driver is larger than the one on the opposite side as shown in Figure 1. The area far away from the driver is shaded in red. Target Figure 1. Examples of side window defrosters and target areas for two current passenger vehicle models

2 These areas are cleared using small side vents designated as side window defrosters. The airflow issuing from these vents is often aimed at the centre of the target area. However, in most ventilation modes available the entrainment from the front foot wells interacts with the side window defroster jets resulting in higher flow diffusion for these jets which are ineffective as illustrated in the laser sheet visualisation images of Figure 2. Further, the strong windshield defroster flow deflected by the A-pillars also lands on the side windows target areas resulting in a detrimental effect on the defrosting action, Figure 3. This paper examines the performance of the side window defrosting and demisting performance on full-scale models of current passenger vehicle models. Comparisons between the experimental and computational results are made between the velocity vectors and contours for each side window. Also a thermal imaging camera, which records the thermal evolution of the side window flow and a video reorder together with a PC are used to capture, analyse and compare the images obtained and interpret the heat transfer mechanisms at glass level. Finally, the drawbacks of current designs are outlined and recommendations for side window defrosting and demisting enhancements are given. Figure 2. The effect of the entrained foot well flow Figure 3. The effect of the windshield defroster flow on the side window defroster flow on the side window defroster flow 2. PREVIOUS WORK Most of the studies reported in the literature are concerned with the front windshield and the thermal comfort of the occupants inside the passenger cabin. Some researchers have attempted to redress the balance. For example, a study by Fredrik (1989); investigated the insulation of the side windows in order to avoid the discomfort to the driver and passengers. The author states that the air has a very low temperature when it reaches the B-pillar, where it easily deflects and impinges on the driver's/passenger's shoulder. He suggests a radical solution by using double-glazing. The window then consists of two hermetically sealed panes with a gas filled or evacuated space in between. It was then found that the surface temperature is significantly raised some 15 o C, at low ambient temperature. Not only are the heat losses reduced, but mist formation is also prevented. Negative aspects are weight and cost. For future development, the author suggests chemically tempered glass might create possibilities for lighter solutions. Another study is that reported by AbdulNour (1998). Computational Fluid Dynamics (CFD) was used to determine the velocity field in the vicinity of the vehicle windshield due to defroster flow. The primary objectives of this study were to quantify the velocity field in the vicinity of the windshield due to defroster jet flow, understand the flow characteristics, and validate the use of CFD simulation in the engineering design and development process. The

3 computational domain included the defroster nozzle and the entire vehicle interior. Hexahedron cells were used in this model. The boundary conditions were assumed to be a uniform velocity upstream at the inlet to the defroster nozzle and a constant pressure downstream at an assumed flow exit at the rear of the passenger compartment. Hot-wire Anemometry was used to experimentally determine the velocity field in the vicinity of the defroster nozzle jet flow and windshield interior surface. The experimental results were used to verify the integrity of the CFD models and validate the use of the CFD modeling approach. Based on the degree of correlation of the velocity profiles between the CFD simulations and the experimental work, the author concluded that CFD simulation is a valid technique to investigate the airflow characteristics of vehicle defroster and windshield flow field. 3. METHODOLOGY The research strategy adopted here is to understand the prevailing defrosting and demisting problems and to survey existing solutions. This is designed to assist in the formulation of new solutions. Therefore, the investigation relies primarily on full-scale experimental tests and numerical investigations. The ventilation mode scrutinized is the Defrost mode. In this mode, air is discharged through the defroster nozzles located at the base of the windshield and on the IP facing the side windows. The data is acquired using the following transducers: 3.1 Hot Bulb Probe This device is used to measure the velocity and the temperature simultaneously. The principle of this thermal probe is based on a heated element from which heat is extracted by the colder impact flow. The temperature is kept constant via a regulating switch. The controlling current is directly proportional to the velocity. Measurements are taken every 50 mm at a distance of 5 mm from the surface of the side window. The measurement grid is shown in Figure 1 (right hand image). 3.2 Thermal Imaging Thermographic techniques can be used in the laboratory at normal ambient temperature and in real time to determine the temperature distributions at the surface of the side window. In this way, it is possible to optimise the performance of the defroster system at room temperature. Thermography is used to map the temperature contours on the side window outer surface. Thermography, in addition to being non-intrusive, shifts the problem from that of direct measurement of air temperature at a specific point in space to that of determining the air temperature ranges in the vicinity of the windshield. An outline of the experimental apparatus used in this work is shown in Figure 4. A thermal imaging camera, which records the thermal evolution of the side window flow and a video recorder together with a PC are used to capture, analyse and process the images obtained. The thermographic camera is positioned outside the vehicle at a distance of about 3-m in front of the side window so that the lens is perpendicular to the plane of the side window. After turning on the blower of the HVAC system and setting up the thermographic system, thermal maps are recorded in time, at intervals of thirty seconds starting from the moment in which the heating is switched on. 3.3 Laser Sheet Visualisation (LSV)

4 Another complementary flow diagnostic tool used is the Laser Sheet Visualization (LSV) method. This involves the projection of a sheet of laser light (2 mm thick) inside the cabin and the recording of the image of scattering particles on a single photographic negative or on a video film. The light source for the present LSV system is an 18 W, continuous wave, Argon Ion laser. The light source is guided to the position of interest through an optical fiber. A sheet forming optical arrangement is attached to the fiber end. The tracer particles are neutrally buoyant and have good light reflection properties. 3.4 Cold Room Test Facility The windshield defrosting system performance is an important requirement for cold climate operation of a vehicle. The defrosting test is preceded by a soaking phase until all vehicle components are cooled down to 18 o C below zero. An ice layer is then formed on the windshield by a uniform spray of a very fine water mist (0.044 g/cm 2 ). The vehicle and the newly formed ice layer are then soaked for a further minutes before the tests begin (Figure 4). Figure 4. Thermal imaging set-up 3.5 Computational Fluid Dynamics

5 The model of the passenger compartment, including the side windows of interest here were drawn using the AutoCAD package and then imported into the GAMBIT software (FLUENT pre-processor) as an IGES file. Fluent Inc.'s T-Grid code was used to generate the tetrahedral volume mesh. The flow and temperature field on the windows and in the passenger compartment is calculated with the CFD code FLUENT. The basic conservation equations, namely the Navier-Stokes equations, are solved numerically using the finite volume technique as implemented in FLUENT. The turbulent nature of the flow is modelled by using the standard k-ε model with logarithmic law of the wall. Constant Pressure is specified at the inlet to the defroster nozzles and the side duct, where the air is sucked in. At the slot in the back of the compartment the air leaves the cabin. The air flow is considered as steady, incompressible, viscous, Newtonian and isotropic. The computational mesh used was made from 750,000 tetrahedral cells as illustrated in Figure 5. After the initial analysis was run, the mesh was refined to increase density in areas that are shown to be important or with high-pressure gradients and, more importantly, to achieve grid independence. The boundary conditions were established by measuring the pressure drop in a prototype model. The total pressure drop of the system from the defroster nozzles to the rear outlet is equivalent to the pressure difference implemented in the present numerical study. Defroster Nozzle s Outlet Side window demister 4. RESULTS AND DISCUSSIONS Figure 5. Passenger cabin computational grid The main task of this study was to assess present practices in defrosting and demisting techniques. Flow along the front side windows is governed by the side window demist jets and the jets issuing from both ends of the defroster registers. The side window demister jets are aimed at the target areas as shown in Figures 1 and 2. Further, the windshield defroster flow and the front foot wells also affect the side window defroster jets decay and effectiveness. For example the front windshield defroster jets deflect off the A-pillar trim and the front windscreen onto the side glass

6 This impingement reduces the efficiency of the side window defrosters results in the high velocities occurring in areas above or below the designated target areas. Part of this flow succeeds in penetrating far into the back compartment, where it meets the returning flow and a stable vortex thus occurs just behind the B-pillar trim. Each side window demist jet entrains air off its nearest foot well, to maintain its momentum, and thus results in a faster radial spread which undermines the defrosting/demisting action. The performance of the side window defroster nozzles was scrutinised, both in terms of the heat generated at the inner surface of the glass, in order to melt the ice on the outer skin and the convection process to remove the moisture from the inner surface. The first mechanism occurs through heat conduction across the glass whilst the second through the elevation of the temperature of the air in the vicinity of the window and consequently the rising of the air s ability to absorb moisture. Therefore, in addition to the velocity field in the areas and planes of interest, thermography is used to map the temperature contours on the side window outer surface. The velocity field was measured using either the thermal probe or the laser based flow anemometer while the temperature field was obtained using the thermal imaging camera. Figures 6 and 7 illustrate the measured velocity magnitudes on the side window for the Ford Focus and Taurus respectively. It is evident that high intensity flow is in the lower area of the glass (just below the designated areas) whilst Figures 8 and 9 show the corresponding thermographs. The graphs show high temperatures in areas far away from the designated targets and the Figures also show the high velocities resulting from the deflection of the front windshield jets by the A pillar. Close to the side glass, the combinations of the side window demist jet and the windshield defrost vent give another jet-to-jet interaction Figure 6. Velocity contours on the sidewindow (Ford Focus) Figure 8. Thermograph of the side window (Ford Focus) Figure 7. Velocity contours on the sidewindow (Ford Taurus) Figure 9. Thermograph of the side window (Ford Taurus) The entrainment of these jets gives the flow pattern shown in Figure 2. The strong upward flow sucked from the foot well wraps around the core of the demister jet and as a consequence introduces a small amount of swirl. This interferes with the side window demist

7 jet by forcing it to spread and hence reduces its momentum. The side window demist jets entrain air from the foot wells with speeds of the same order of magnitude as themselves. This entrainment, although inevitable, must be controlled in order to eliminate dust and odour being transported all over the cabin. A further scrutiny of the side window flow for the Ford Focus through a parametric numerical study reveales the extent of the influence of the cabin bulk flow on the side window defrosting/demisting mechanism. Figure 11 depicts a recirculation generated by the forward and returning cabin flow meeting on the plane of the side window. This is undesirable and detrimental to the defrosting/demisting process. Other small scale eddies generated by the shear between the fast front windshield defroster flow and the slow air in front of the driver are clearly depicted in Figure 13. Figure 10. Predicted velocity magnitude on th side window (Ford Focus) Figure 12. Predicted temperature contours on t side window (Ford Focus) Figure 11. Predicted velocity vectors on the si window (Ford Focus) Figure 13. Driver side window demist flow (Ford Focus) Figures 10, 11 and 12 represent the predicted velocity magnitudes, temperature contours and the velocity vectors map on the side window. The agreement between the numerical simulation and the measured data shown in Figures 6, 7, 8 and 9 is fairly good. This is also in agreement with the velocity distribution on the side windows shown in Figures 14 and 15. The maximum discrepancy recorded in the area of interest between the measurements and predictions is 13%. These are the passenger and driver side respectively for a left hand drive vehicle. Both Figures show that the high velocities are recorded below the target area and therefore the defrosting/demisting process needs optimising. This should mean a control over the flow prevailing within the cabin, the position of the defroster nozzles and the intensity of their discharge. Consideration should also be given to the interaction between the discharge and the entrainment. This is invariably a function of the local topography and the trimmings. The side window demist function can also be optimised by re-adjustment of side window registers vane and duct angles

8 Figure 14. Velocity contours on the passenger side of a left hand drive vehicle Figure 15. Velocity contours on the driver side of a left hand drive vehicle 5. CONCLUSIONS This study has investigated the fluid flow and heat transfer on the side window of current fullscale vehicles. The drawbacks of current practices have been examined and ways to improve the defrosting and demisting process have been outlined. Most of the tests show that the defroster/demister flow is aimed below the target area. Therefore, the side window demist function can be optimised by re-adjustment of vane/duct angle. This has been

9 practically implemented in the case of the Ford Focus when 14 o resulted in a cleared target area. upward inclination has ACKNOWLEDGMENT The support of Visteon Climate Control Systems (Plymouth, Michigan, U.S.A.) is kindly acknowledged. REFERENCES [1] AbdulNour, B. S Numerical Simulation of Vehicle Defroster Flow Field, SAE Paper No [2] Carignano M. and Pipppione E., 1990, Optimisation of Windscreen Defrosting for Industrial Vehicle via Computer Assisted Thermographic Analysis, SAE Paper No [3] Dugand M. M. and Vitali D. F., 1990, Vehicle Internal Thermo-Fluid Dynamics; Experimental and Numerical Evaluation, SAE Paper No [4] Fredrik N., 1989, "Insulating Glazing in Side Windows" SAE Paper No [5] Nasr, K. J. and AbdulNour B. S., 2000 "Defrosting of Automotive Windshields: Progress and Challenges", Int. J. of Vehicle Design, Vol. 23, pp

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