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GVSU EGR 468 - Transient Spatial Response of Various Solids

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EGR 468 Heat TransferLab Exercise 1Transient Spatial Response of Various Solids(Using Thermocouples)Purpose: The purpose of this lab is to experimentally determine the transient spatial temperature response of different solids to a sudden change in boundary conditions. Lab Exercise: We will be conducting an experimental study of the difference between how a water-based solid and a metal solid respond to a sudden change in boundary conditions. In each case the solid will start at uniform atmospheric temperature. Place the solid into a boiling water bath or an ice bath and observe the temperature response as a function of time and position. In your report answer the following questions: - What is the governing equation for temperature vs time and position in the solid? - What are the differences between the two solids based on the governing equation and basedon your experimental results? - What physical properties do you suspect are responsible for any differences observed? - Do the results make sense? - Is there any advantage to using the boiling water bath vs. the ice water? - When you lower the solid into the water bath you are changing the boundary condition. Is this closest to a constant temperature or to a convection boundary condition? Offer evidence to support your answer.Background on Thermocouples: Temperature can be measured using any physical property that changes in a highly reproducible way as temperature changes. Engineers are most likely to come in contact with one of six types of temperature sensors: thermocouples, resistance-temperature detectors (RTD’s and thermistors), infrared detectors, bimetallic devices, liquid expansion devices, and change of state devices. In this list thermocouples are perhaps the most commonly used sensorsin engineering applications. They are relatively inexpensive, they work in chemically harsh environments, they are rugged and durable, and they work over a very large temperature range.In 1821 T.J. Seebeck discovered that an emf exists across a junction formed from 2 dissimilar metals. The emf which he observed was later found to be a result of primarily the Peltier effect in which an emf is formed as a result of the contact of the 2 metals and the temperature of the junction. This effect forms the basis for a thermocouple. The figure below shows a very simple thermocouple circuit in which 2 junctions are formed using metal A and metal B at junction temperatures T1 and T2. The net emf flowing in this circuit will be the difference between the Peltier emf's occurring at the 2 junctions. If the junction temperatures are identical, no emf will be observed; under all other temperature conditions a highly reproducible emf which is a unique function of the temperature difference is observed. Two junctions will always be required in a thermocouple circuit; generally one will sense the unknown temperature while the other junction (known as the reference junction) will usually be maintained at a known temperature. Thermocouple tables giving emf as a function of temperature for various metallic combinations have been developed based on the ice point as a reference temperature.Two laws governing thermocouple circuits are particularly important in application.1. The Law of Intermediate Metals states that the insertion of an intermediate metal into a thermocouple circuit will not affect the net emf provided that the two new junctions introduced by the third metal are at identical temperatures. This law allows us to use lead wires made of a third (and less expensive ) type of metal as long as the new junctions formed are at the same temperature.This can generally be assured by making both lead wire junctions in the same place. ( not shorting them of course!) This law also allows us to use solder in a thermocouple junction. Welding the junction is preferred. If a weld is not possible, you can simply twist the wires, however solder will increases the mechanical strength of the junction and will assure electrical continuity.2. The Law of Intermediate Temperatures states that if a simple thermocouple circuit develops an emf, e1, when its junctions are at T1 and T2 respectively, and if the same circuit develops an emf, e2, with its junctions at T2 and T3, it will develop an emf, e3= e1+e2 , when the junctions are at T1 and T3. This law allows us to use the tables which are based on the ice point as the reference temperature even when the actual reference temperature is some other value. (Let T2 be the independently measured reference temperature and T3 be the unknown temperature, then e1 is the emf from the table for the reference temperature relative to the ice point, e2 is the measured emfat the unknown temperature T3 relative to T2, e3 can found in the table and represents the emf resulting from the unknown temperature relative to a reference temperature at the ice point.)In the lab we will be using type K ( chromel(+) vs. alumel(-)) thermocouples. This combination offers high sensitivity and also nearly linear emf vs. temperature response over a 0˚-1000˚C range. This covers the range expected in our lab. You may notice that the reference junction seems to be missing. We use a hardware compensated system that provides an output voltage in millivolts that is equal to the temperature in the selected system (˚C or ˚F). Note that this unit must be used with atype K thermocouple only.


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