Aircraft gasper jet influence on local thermal comfort

Transcription

1 Indoor Air 2008, August 2008, Copenhagen, Denmark - Paper ID: 702 Aircraft gasper jet influence on local thermal comfort Fernando Stancato 1,* and Arlindo Tribess Polytechnic School of the University of São Paulo, Department of Mechanical Engineering, Brazil * Corresponding fernando.stancato@poli.usp.br SUMMARY Nowadays many different researches are occurring to develop a personal ventilation system in aircrafts. While the aerospace industry will take some time to implement these new concepts in new projects, the gasper use is still the only way the occupants have to improve their local thermal comfort. A numerical CFD study is presented in order to find the relationship between the initial jet temperature and mass flow to the local thermal comfort on the head, chest and face. Typical regional airplane geometry was used with two passengers seated. The passengers were modeled with mannequins with body and arms. First, it was studied if the gasper jet has influence on only one passenger or if affects the others. It was found that the gaspers can influence the local thermal comfort with minimum influence on the others. It was found good correlation on the experimental and numerical central velocity gasper jet. As the jet was well modeled, it was calculated the equivalent temperatures on the head, chest and face with different initial jet temperatures. It was found that the three body parts presents different dependence on the initial jet temperature do to the different distances of the gasper valve and thermal cloth resistances. Lower temperatures can cause a higher discomfort than higher temperatures. The respective limits were 19 o C and 50 o C for the initial jet temperature for a gasper mass flow of kg/s. KEYWORDS Local thermal comfort, Non isothermal jets, CFD, Indoor flow, Aircraft internal flow INTRODUCTION One of the most difficult tasks to the aerospace comfort engineer is to provide comfort to all the aircraft passengers. The restricted space between the occupants, the small pitch between the seats, the variability of the heat sources, the asymmetry of the air temperature and velocities are some of the obstacles to provide an acceptable thermal ambient. Another obstacle is the personal differences that make the scenario even worst. The typical spectrum of the today passengers include frequent and new flyers. This may led to different reactions during the flight (scare, panic, relax, etc) that may led to different metabolism that would impact on their thermal feeling. Added to this, are the passenger s different clothing options. All these facts are leading to recent initiatives to promote a personal thermal climatic adjustment as can be seen in the high class automotive vehicles. Some researches to develop an aircraft personal thermal climatic adjustment can be seen in recent publications (Gao and Niu, 2007; Melikov et al., 2007). As the aerospace industry is known to be very conservative, these researches may take some time to reach the nowadays passengers. In this way, the standard gaspers that have been used since the beginning of the pressurized airplanes are still the only way the passengers have to improve their local thermal comfort.

2 The gaspers are the common name the interior aerospace engineers call the air diffusers that usually stays over the head of the passengers. They usually permit the passengers to adjust the direction and the velocity of the air jet. Traditionally the gasper valve has an internal needle that can close the outlet or let the flow pass in a 10 mm hole making a very directive air jet. Usually the air for the gaspers comes from the air conditioning system. A logic in the system valves may be set not permitting gaspers air temperatures too high or too low. The first would not permit the cooling effect and the second could cause cold discomfort. This paper will investigate two questions: a) If gaspers can be used as a personal ventilation system. The idea is to investigate if the gasper is personal. In other words, if it can improve the local thermal comfort without disturbing the other passengers. b) If the above question could have a positive answer, which would be the limits of the gasper inlet air temperatures. To answer these questions a CFD study was done with manikins seated in a typical regional jet interior configuration. The ISO standard methodology was used as a criteria to evaluate the comfort. THERMAL COMFORT EVALUATION USING EQUIVALENT TEMPERATURE A criterion for the evaluation of thermal environments using equivalent temperatures was introduced by Wyon et al. (1989) and Holmer et al. (1995). Based on the evaluation of a panel of 20 people the ideal equivalent temperatures and their limits of tolerance for 18 segments of the passengers' body were carry out in a vehicle in typical conditions of summer and winter, as presented in Figure 1. The same limits were adopted in the ISO standard (ISO, 2006). Figure 1. Ideal profile for the equivalent temperatures and limits of tolerance for summer condition (Nilsson, 2004). MANIKINS To evaluate equivalent temperatures the best method is that using thermal manikins or manikins with heated sensors (Gameiro, 2002, Murakami et al., 1997). Nilsson (2004) presents a detailed study of the use of thermal manikins. A thermal manikin should have some characteristics to be used in evaluation of thermal comfort inside vehicles. A typical thermal manikin that doesn't simulate sweat is built with a structure in aluminum covered by plastic foam. Each independently heated area is covered by electric resistances that later are covered by a plastic resin. Sensors of temperature are mounted on the manikin's surfaces.

3 For the control of the temperature and thermal heat flow there are, basically, three control methods: constant superficial temperature, constant heat flow and Fanger s skin comfort temperature equation: a) In the first method a control system maintains the skin superficial temperature constant, typically 34 o C. b) In the method of constant heat flow a control system doesn't exist; the tension and voltage applied to the resistance of each part of the manikin's are maintained constant. c) The third control method uses the equation of Fanger s skin comfort temperature modified to discount the heat lost by the skin water evaporation. MANIKIN S CALIBRATION To accomplish the evaluation of thermal comfort conditions using manikins and the concept of equivalent temperature, the manikins should be calibrate in a climatic chamber with air velocity close to zero and with the same and uniform temperatures of the air and wall surfaces. That procedure is necessary to apply the concept of equivalent temperature. Doing it the air and wall surfaces temperatures will be the known equivalent temperature, t eq, typically equal to 25 o C for summer and 21 o C for winter (Nilsson, 2004). In this calibration procedure the coefficients of heat transfer, h cal, for each segment of the manikin, will be determined using equation (1): h cal = t s q t eq (1) where h cal = calibration heat transfer coefficient [W/m 2. o C] t eq = equivalent temperature (25 o C or 21 o C) [ o C] t s = surface temperatures of the manikin segment [ o C] q= heat flux of the manikin segment [W/m 2 ] The clothes used in the calibration process are the same ones used in the real environment and the heat changed by each segment of the manikin is also the same in the calibration environment and in the real environment. Once obtained the calibration heat transfer coefficients, h cal, the measurements of the new surface temperatures, t s (considering constant heat flow method) or the new heat flows, q (considering constant skin temperatures) in the real environment are performed and the equivalent temperatures, t eq, are calculated using equation (2): t eq = t s q h cal (2) NUMERICAL MANIKINS With the development of advanced CFD codes for the simulation of ventilated environments, several researchers began studies to model the human body numerically. Nilsson (2004) in his work lists nine physiological models of the human body and twelve methods of thermal manikins construction in the last ten years. These numbers show the great interest that this topic has been receiving lately.

4 The models vary greatly in complexity. Some possess physiological and psychological models and other only evaluate heat flows or temperatures prescribed in the manikin segments. In the first type there are three-dimensional models that possess a model for the thermal changes with the external environment and a model for the heat changes in the interior of the human body (thermoregulation system) (Murakami et al., 1997;Maué et al. 1997). The other types of three-dimensional models don t include physiological and psychological models, considering only heat flows (Brohus and Nielsen, 1996; Bjørn, 2000) or prescribed temperatures (Nilsson, 2004) for the manikin's surfaces. CONSTRUCTION AND CALIBRATION OF THE NUMERICAL MANIKIN In order to make the study it was used the digital thermal manikin methodology (Nilsson, 2004; Stancato et al., 2007). The purpose of the digital calibration is the same of the experimental thermal calibration: to calculate the global heat transfer coefficient of each manikin part. These coefficients will be used later to calculate the equivalent temperature at the gaspers air jet thermal comfort evaluation. First a digital manikin was calibrated in a cubic chamber (3x3x3 m) with floor displacement ventilation. The grid had 885,000 elements. The manikin had constant skin temperature of 34 o C and local summer cloth resistances (0.178, and m 2. o C/W respectively for scalp, face and chest areas). The simulations were done with the Fluent CFD code (FLUENT, 1998). Air was modeled as ideal gas with the realizable k-ε turbulence model. Radiation heat exchange was modeled with the discrete ordinates method. After numerical good convergence it was calculated the calibration heat transfer coefficient, h cal, to be used in the calculation of the equivalent temperature. a) b) Figure 2. a) Geometry of the computational manikin with 18 segments b) The calibration chamber and manikin grid. Calibration boundary conditions The boundary conditions set for the numerical calibration of the manikin are: Air supply - The air supply is done through the floor and was modeled as perfect gas with inlet velocity of 0.03 m/s parallel to the lateral walls, and temperature of 25ºC. It was considered turbulence intensity of 5%, hydraulic diameter of 5.3 m.

5 Air exhaust - The air exhaust is done by the ceiling and modeled as pressure outlet with gauge pressure equal zero. Cabin walls - The black body temperature method was used to specify cabin wall temperatures: 25ºC. Emissivities equal 0.95 were considered. Manikin - The temperatures of the manikin surfaces were maintained constant in 34ºC. For that different thermal resistances for each body zone were informed. A "cotton material with different values of thermal resistances, using different clothes thickness, was considered. GASPER JET INFLUENCE ON THE NEIGHBOUR PASSENGERS The gasper air jet influence was done using typical regional airplane geometry. Symmetrical cabin geometry was used with two passengers seated side by side with an empty seat in the front. The gasper was located at the overhead bin and had a 10 mm inlet diameter. A grid was generated using a fine refinement in the gasper jet path to the passenger near the wall (passenger A). The grid had 1.5 million tetrahedral elements. a) b) Figure 3. a) Manikin s position inside the cab. b) Cut in grid with a plane passing in the center of the head. Notice the fine grid between the manikin s A head and the gasper inlet. The simulations were done with the Fluent CFD code (FLUENT, 1998). Air was modeled as ideal gas with the realizable k-ε turbulence model. Radiation heat exchange was modeled with the discrete ordinates method. Aircraft walls were set with outside skin temperature of 34 o C with typical aircraft insulation. The floor was set with 5 o C with its thermal resistance. Two air inlets provide the incoming air at the temperature of 23 o C and constant velocity of 0.88 m/s. One inlet is between the overhead bin and the ceiling and the other is on the inferior part of the overhead bin near the side wall. First, it was studied if the gasper jet has influence on only one passenger or if affects the others. It was chosen a typical gasper mass flow of kg/s and a high temperature of 50 o C in order to create a high non isothermal jet.

6 RESULTS As can be seen in the next figure the grid density for the gasper jet had enough resolution to capture the velocity and temperature gradients. It was compared the numerical and experimental results of the jet center air velocities and good correlation was found. Figure 4. Cut in the grid in a jet axis symmetrical plane. The volumetric grid was colored with the velocity pallet. It was calculated the equivalent temperatures (t eqs ) of the scalp, face and chest, that would be the most affected manikin parts. Two simulations were performed: one with the gasper valve open in the manikin A head/face direction and another with the gasper valve closed. Follows the results: Table 1. Equivalent temperatures on the scalp, face and chest on the mannequin A and B with and without gasper air jet. Equivalent Temperatures ( o C) Parts Manikin A Manikin B Scalp 24,42 23,89 Without gasper jet Face 24,87 23,57 Chest 24,34 24,09 Scalp 26,35 24,26 With gasper jet Face 21,20 24,45 Chest 25,90 24,92 It can be seen that, without the gasper jet, the manikin near the wall (A) presented higher t eqs (0.7 o C in average) than manikin B. This may be due to the lower air circulation near the walls to these parts. It could be noticed that the manikin near the corridor had its temperature increased about 0.7 o C in average when the gasper valve was opened. This is an indication that the neighbour passenger felt a little the hot air coming from the gasper, but without much notice or annoyance. It should be said that it is not very common the occurrence of these high temperatures in the low pressure air conditioning ducts.

7 The manikin that had the gasper jet directed to his head had the head and chest t eqs increased in 1.6 o C and the face temperature decreased in 3.7 o C. This fact happened do to the high convective heat transfer on the face surface. With this it can be seen that even high temperature air gasper jets can promote some local (face) thermal comfort to passengers with some global thermal discomfort and that it would affect very little the neighbour passenger. UPPER AND LOWER GASPER AIR INLET TEMPERATURES LIMITS Once it was found that the gasper temperature does little impact on the neighbour passengers an investigation was done to determine the upper and lower gasper inlet temperature. In this study it was used the same mesh of the previous study. It was calculated the mean equivalent temperature of the same previous manikin (A) parts with the inlet temperatures of 60, 50, 24, 19 and 5 o C. Follows the equivalent temperatures for the manikin A parts. Table 2. Equivalent mean temperatures for different gasper inlet temperatures. Manikin A parts ( o C) Scalp Face Chest Inlet temperatures ( o C) No gasper jet It could be seen that the most sensitive part for the changes in the inlet gasper temperature was the face do to its unclothed situation and their relative position to the gasper jet. As the main purpose of the gasper in the aircraft is to bring the passengers upper part (scalp, face and chest) to the cooler limits of the ISO standard comfort zone, it was adopted the lower limits of the cold but comfortable zone for these parts (10.9 o C) and the upper limits of the neutral zone (26.7 o C). Considering this, the lower and the upper limits for the gasper inlet air temperature shall be 19 o C and 50 o C. CONCLUSIONS The gasper valve inside the aircraft can still be used as a personal ventilation system. Its use has a minimum impact on the neighbour passenger. It is recommended that the air inlet temperature should be kept between 19 o C and 50 o C in order to keep its local cooling effect. ACKNOWLEDGEMENTS Arlindo Tribess wish to acknowledge the Brazilian National Research Council (CNPq) for the financial support received.

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