Measurement of the convective heat transfer with wind : Developing and testing an Earth Scaled Atmospheric Temperature Sensor

University essay from Luleå tekniska universitet/Institutionen för system- och rymdteknik

Abstract:    The HAbitability, Brine Irradiation and Temperature Package (HABIT) instrument of the ExoMars Surface Platform will investigate the present day habitability of Mars at the near surface environment. This instrument includes three Atmospheric Temperature Sensor's (ATS's) which are similar to the ones previously used on the Rover Environmental Monitoring Station (REMS) of the Mars Science Laboratory (MSL) rover, that has now been operating on Mars for more than four years. The ATS of REMS is only used to provide the air temperature, however on HABIT it will be used furthermore to provide information about winds and heat transfer at the surface of Mars. The retrieval method needs to be further investigated and validated. This master thesis is aimed at three goals: 1) the development and testing of an Earth Scaled Atmospheric Temperature Sensor (ESATS) to test the retrieval concept; 2) the validation with other Earth-based standard wind sensing technologies under outdoors uncontrolled conditions; and 3) the analysis of the existing observations of the ATS of REMS on Mars to get a better understanding of its expected future performance on HABIT once it operates on Mars. The ESATS is an up-scaled semi-autonomous prototype version of an ATS which consists of a rod of different size and material to those that are used on REMS and will be used on HABIT. The rod shall be heated from the base where it is attached to. The temperature profile shall be measured at three different measurement points. All these temperatures are different from the one of the atmosphere to which the rod is exposed to. The temperature profile along the rod changes depending on the air temperature, air density and the wind speed because of the convective heat transfer. A preliminar analysis is used to define what is the ideal length of the rod, and what is the material that is best adapted for this experimental prototype. Since the air density is needed to retrieve the wind speed, the pressure will be monitored as well. In parallel, a second wind measuring technique based on the dynamic pressure changes detected in a Pitot tube is used as control. The measuring campaign is subdivided in several stages: 1) The first part will take place in a laboratory, where the system is exposed to static conditions with no heating and no wind, which means that there is no forced convection caused by wind. In this setup all sensors are calibrated against one another and with help of a reference resistor the temperature sensors are also calibrated to 273.15K. Additionally different Operational Amplifiers (OpAmps) will be used to observe how the noise level is affecting the measurements, so that the best one will be used in the end. 2) Furthermore, the best position to place the intermediate temperature sensor is investigated by testing one of the rods with different locations of the middle temperature sensor. 3) Next, also within the laboratory environment, the different rods are used to obtain the temperature profile and retrieve the air temperature and heat transfer values, solving the equations that describe the heat transfer problem under static conditions. 4) The second part of the measurement campaign will take place outdoors, where the ESATS is exposed to forced convection due to wind. In this setting first the influence of the Sun on the system is measured, as it is important to know, if the measurement can be performed when the illumination conditions change. 5) Next, the system is tested with the 50cm rod in long term tests with the reference measurement of a commercial weather station (HOBO) next to it. With the data obtained the convective heat transfer method is used and the retrieved wind speed is compared to the one received from the HOBO. Finally, to get a better understanding of its expected future performance on HABIT once it operates on Mars, the data of the ATS of REMS is used to perform the wind speed retrieval for Mars and to compare it with the data received from the REMS wind sensor. It is only operating during daytime and has still difficulties to retrieve a precise wind speed. The measurement campaign has given several information about ATS in general. First it was decided to place the temperature sensor in the middle at 1/4 of the rod length, which is optimal for the retrieval process and which is also coincident with the one chosen for HABIT and REMS. The measurements in the laboratory are providing good and constant temperature profiles with the chosen setup which correspond with the one expected from the equations that describe the heat transfer problem in a long rod. On the other hand, it is not possible to calculate a valid ambient temperature for the short rods, which is because of an overheated boundary layer around the rods due to the heating. For this reason, it is recommended to use the longest rod in the lab. For outside testing the influence of the Sun could be confirmed and was affecting the measurements of the copper rod. During the time span where the prototype was in the Sun, it was not possible to get any reasonable results. The next measurement campaign was defined in a shadowed area with diffuse light only. Finally, the tests of exposure to dynamic changes over time are in excellent agreement with the ones provided by the HOBO station and can even give a better resolution and sensitivity to small changes of wind magnitude. This prototype has confirmed experimentally, that under Earth conditions, this method can be used to retrieve the wind speed. Finally, the Martian data of the REMS ATS are analyzed and the comparisons suggest that the method is sensitive to wind changes on Mars as well, and shows better time and magnitude resolution than the existing REMS wind sensor. This confirms that this method can be successfully used for the HABIT sensor. In this work the methodology that shall be used on HABIT to retrieve the convective heat transfer and wind on the surface of Mars is validated for Earth outdoors conditions. It is recommended to verify the obtained results with tests in a wind tunnel and to see how the system will behave with a higher heating and different rod materials. Furthermore, the setup should be tested in a way which makes it possible to determine the different directions of the wind.

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