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Fast Response Thermocouple Seminar Report

Fast Response Thermocouple is to measure temperature of surfaces experiencing rapid temperature fluctuations.
Engineering applications frequently require the measuring temperature variation of surfaces exposed to a fast changing heat flux. Typical examples of surfaces experiencing rapid temperature fluctuations include engine cylinder wall; hot-rolled metal sheets quenched by water-sprays; and dies used for casting and metal parts. In this report, a study is conducted on thermal barrier coatings (TBCs) in an aluminum alloy spark-ignition engine. For many years, the thermocouple has been the sensor most commonly used to measure surface temperatures. However, there is no consensus about the optimal thermocouple design or even general configuration that best suits fast response engine applications.

The primary consideration when selecting or designing a surface thermocouple is accuracy at measuring ‘true’ temperatures, which is the temperature that would exist if the thermocouple was not present. For the surface thermocouple, this includes accurate measurement of average temperatures and temperature swings, while minimizing delays in response time. Additionally the thermocouple should be robust and simple to fabricate and also to install. To measure temperature variations accurately requires a thermocouple with a response time of only a few micro-seconds.

Finite Element Analysis (FEA) was used to explore the accuracy issue in1989 to model a 0.452mm diameter co-axial thermocouple embedded in an aluminum piston. A co-axial thermocouple is essentially an insulated wire of one thermocouple element housed within a tube of a second element. A thin metal junction forms the electrical connection between the wire and tube at the thermocouple tip. It was found that type K and type J thermocouples overestimate the magnitude of true temperature swings by 59% and 69% respectively. Type K thermocouples are made of chromel (90% nickel and 10% chromium) and alumel (95% nickel, 2% manganese and 1% silicon). It’s the most common type of thermocouple used. It has a sensitivity of 41µV/ºC. It can detect temperature in the range -200ºC to +1250ºC. It also contains a magnetic material nickel. Type J thermocouples are Iron-Constantan thermocouples having a restricted range of -40ºC to +75ºC. It has a sensitivity of 55µV/ºC. The difference however in the magnitude of true temperature swings is due to the substantially different thermal properties of the aluminum substrate compared to the thermocouple materials. Aluminum’s thermal conductivity is at least three times greater than that of every metal in type K and type J thermocouples, and aluminum’s thermal diffusivity is at least four times greater than that of the thermocouple metals. As such, heat does not diffuse as readily through the thermocouple and its junction experiences larger temperature swings than the more conductive aluminum substrate. This difference in thermal properties also introduces lateral heat conduction between the thermocouple and the substrate. Similar coaxial thermocouples have been used in recent engine heat transfer studies.

A different coaxial thermocouple was developed by scientists Furuhama and Enomoto and by their analysis, was found to be very accurate in measuring temperature swings. The design featured 3 mm aluminum cylinder with a 0.17 mm hole drilled through to house an insulated constantan wire. Interestingly, copper was used to form the junction and its thickness was optimized to provide sufficient heat conduction from the wire tip to the aluminum to allow for temperature equalization. The optimal copper thickness was found to be 7–10 lm. However, the 0.17 mm diameter hole is too small to be drilled in a standard machine shop and therefore this exact design was not feasible. The eroding thermocouple is another style of thermocouple that is frequently used for engine measurements. The design utilizes two thin ribbons of thermocouple element, insulated and housed within a metal sheath. The electrical connection between the two elements is formed by abrading the surface. Despite the small size of thermocouple elements, analysis using FEA shows that the temperature at the junction is significantly higher than that of the sheath during transient heat flux.

An alternative surface thermocouple was developed by Heichal et al. to measure temperatures of liquid droplets impacting a metal surface. The steel substrate was used as one of the thermocouple elements, and a 0.25 mm diameter constantan wire was the second element. The wire was cemented in a 0.57 mm diameter hole through the steel, and the junction was formed at the surface with an electrically conductive film. The substrate being one of the elements, the design has the potential to produce accurate true surface temperature measurements. However, it is found that modifications were required to improve its robustness and accuracy for engine use. This report describes the modifications made to the Heichal et al. thermocouple to improve it for measuring temperatures of aluminum combustion chamber surfaces. The process included the use of FEA to optimize and evaluate the design. Ultimately, the thermocouple was tested in an SI engine.

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