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</o:shapelayout></xml><![endif]--></head><body lang=EN-CA link="#0563C1" vlink="#954F72"><div class=WordSection1><p class=MsoNormal>Dear Friends of Experimentation<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>This is your chance to participate in developing an important correction to one metric included in nearly all stove performance test methods.<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>Purpose of the experiment: to determine the heat transfer efficiency between a fire and a pot.<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>Background: It is well known that the heat transfer efficiency metric is of interest to stove developers and that the heat transfer efficiency has been reported for more than 30 years as a proxy for fuel consumption (which it is not). In order to provide a better answer to the stove designer who often seeks to maximise the performance of this metric, we have to quantify two things.<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoListParagraph style='text-indent:-18.0pt;mso-list:l1 level1 lfo1'><![if !supportLists]><span style='mso-list:Ignore'>1.<span style='font:7.0pt "Times New Roman"'> </span></span><![endif]>The rate of heat gained by the pot+water per unit time, plus the rate at which heat is gained and lost from the pot in ways that are difficult to measure directly.<o:p></o:p></p><p class=MsoListParagraph style='text-indent:-18.0pt;mso-list:l1 level1 lfo1'><![if !supportLists]><span style='mso-list:Ignore'>2.<span style='font:7.0pt "Times New Roman"'> </span></span><![endif]>The heat available from the fire per unit time.<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>The ‘unit’ of time could be 20 minutes or any other reasonably long interval.<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>Part 1 above has two portions, the heat gained by the water and the heat gained by the pot. Several test methods include the thermal mass of the pot so I will go with that version (like the Indian Standard) because pots vary a lot in mass which affects performance both when the temperature is going up or down.<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>Section 1 of Part 1 is calculated from the temperature change in the water in the pot. The same is calculated for the mass of the pot and lid (and thermocouple if it is included). This we have been doing for a long time so there is no big discovery to be made.<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>Section 1 Part 2 is the experiment you are invited to conduct and report. How do we determine the heat gained by the pot and lost to the environment? A discussion a few months ago between Prof Annegarn and me led to the following experiment which you can do at home using any pot and amount of water you choose.<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoListParagraph style='text-indent:-18.0pt;mso-list:l0 level1 lfo2'><![if !supportLists]><span style='mso-list:Ignore'>1.<span style='font:7.0pt "Times New Roman"'> </span></span><![endif]>Select a pot to use in the experiment. Record the dimensions, mass, material and if possible it should not have a single long plastic handle – just metal or small handles.<o:p></o:p></p><p class=MsoListParagraph style='text-indent:-18.0pt;mso-list:l0 level1 lfo2'><![if !supportLists]><span style='mso-list:Ignore'>2.<span style='font:7.0pt "Times New Roman"'> </span></span><![endif]>Cut a piece of white foam plastic such as Styrofoam into a circle that is the same as the outside diameter of the pot. It should be at least 12.7mm thick. Thicker is no problem.<o:p></o:p></p><p class=MsoListParagraph style='text-indent:-18.0pt;mso-list:l0 level1 lfo2'><![if !supportLists]><span style='mso-list:Ignore'>3.<span style='font:7.0pt "Times New Roman"'> </span></span><![endif]>Fill the pot with water to between 66% and 85% of its capacity. Record the volume of water used either using a measuring cup or scale. <o:p></o:p></p><p class=MsoListParagraph style='text-indent:-18.0pt;mso-list:l0 level1 lfo2'><![if !supportLists]><span style='mso-list:Ignore'>4.<span style='font:7.0pt "Times New Roman"'> </span></span><![endif]>Record the ambient in the room temperature using the thermocouple or thermometer then put it into the water 1/3 of the diameter away from the pot wall, 4-5 cm above the bottom. This will later be used to measure the water temperature.<o:p></o:p></p><p class=MsoListParagraph style='text-indent:-18.0pt;mso-list:l0 level1 lfo2'><![if !supportLists]><span style='mso-list:Ignore'>5.<span style='font:7.0pt "Times New Roman"'> </span></span><![endif]>Place the pot of water on a heat source and heat it until the contents are about 80 degrees C. The lid should always be on to prevent virtually all evaporation.<o:p></o:p></p><p class=MsoListParagraph style='text-indent:-18.0pt;mso-list:l0 level1 lfo2'><![if !supportLists]><span style='mso-list:Ignore'>6.<span style='font:7.0pt "Times New Roman"'> </span></span><![endif]>Remove the pot of water from the heat source and place it centrally upon the insulating foam disk. Record the temperature and time.<o:p></o:p></p><p class=MsoListParagraph style='text-indent:-18.0pt;mso-list:l0 level1 lfo2'><![if !supportLists]><span style='mso-list:Ignore'>7.<span style='font:7.0pt "Times New Roman"'> </span></span><![endif]>Record the temperature and time at 30 second intervals for the next 25 minutes. This will generate 50 pairs of readings during which the water temperature will drop at a rate determined by the masses, surface finish, colour etc.<o:p></o:p></p><p class=MsoListParagraph style='text-indent:-18.0pt;mso-list:l0 level1 lfo2'><![if !supportLists]><span style='mso-list:Ignore'>8.<span style='font:7.0pt "Times New Roman"'> </span></span><![endif]>You can carry on longer if you want because you may be interested to see how the cooling rate changes with temperature.<o:p></o:p></p><p class=MsoListParagraph style='text-indent:-18.0pt;mso-list:l0 level1 lfo2'><![if !supportLists]><span style='mso-list:Ignore'>9.<span style='font:7.0pt "Times New Roman"'> </span></span><![endif]>Enter the data into a spreadsheet with column A being the time and column B being the temperature.<o:p></o:p></p><p class=MsoListParagraph style='text-indent:-18.0pt;mso-list:l0 level1 lfo2'><![if !supportLists]><span style='mso-list:Ignore'>10.<span style='font:7.0pt "Times New Roman"'> </span></span><![endif]>Enter also the other relevant information about the ambient temperature, the pot mass, dimensions, appearance etc. Include one small photo in case the shape is unusual or the lid something other than flat. If you know your elevation, or the local barometric pressure and humidity, include that too. If you perform the experiment more than once, create a new identical tab for each replication. You might use different pot sizes and amounts of water. <o:p></o:p></p><p class=MsoListParagraph style='text-indent:-18.0pt;mso-list:l0 level1 lfo2'><![if !supportLists]><span style='mso-list:Ignore'>11.<span style='font:7.0pt "Times New Roman"'> </span></span><![endif]>Send it to me by email or share it with the group.<o:p></o:p></p><p class=MsoListParagraph style='text-indent:-18.0pt;mso-list:l0 level1 lfo2'><![if !supportLists]><span style='mso-list:Ignore'>12.<span style='font:7.0pt "Times New Roman"'> </span></span><![endif]>The same experiment can be performed with a boiling pot but it requires that you put the pot and insulation on a scale to observe the loss of mass by evaporation for each reading.<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>Calculations: The calculated value will be the rate of heat loss from the pot. The insulation under the pot represents the fire, in that no heat will be lost downwards (as is the case when the fire is really there). The result of the calculation is the rate of heat loss in Joules per second [Watts]. <o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>This heat loss is constantly taking place during cooking. The heat is gained and lost with no benefit in terms of cooking, however that is not the point. Trying to calculate the ‘thermal efficiency’ at high or low power usually means ignoring this energy loss and giving a ‘net heat gained’ value. It sounds good, but the result is misleading, and in the case of low power, very misleading. The question is, <i>how misleading</i>? That is what the experiment seeks to assess.<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>As the loss is constant for a simmering pot, the loss might be a small or large value relative to the energy lost due to evaporation. Suppose it constitutes most of the heat gained by the pot. In that case using the evaporated water only as a measure of heat gained means not counting the majority of the heat transferred rendering the result basically meaningless. In an extreme case one could put in only enough energy to substitute for losses from the pot without every evaporating anything at all. A simple calculation would say the efficiency was 0% because no work was done. <o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>Suppose the fire is operating at 800 Watts. Suppose the heat loss rate from the pot is 400 Watts. Suppose the evaporation rate counts for another 200 Watts. The ‘standard’ calculation says 200/800 = 25% heat transfer efficiency (and that really is the calculation used). If you add the pot losses to the evaporative loss it is 600/800 = 75% heat transfer efficiency which is the real value.<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>By adding the heat loss rate to a high power boiling phase using the heat loss rate at the temperature [(Initial+Final temp)/2] (or other more accurate method of choosing a temperature) the heat transfer value would be more accurately reported. It would not depend on the power level of the fire, meaning the heat transfer efficiency at high and low power, or mixed, can be accurately reported.<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>The heat loss comparison between shiny and blackened pots will be very interesting to see. Surface roughness will play a factor. We are not trying to calculate the loss, remember, we are trying to measure it. The data will come from the measurement of the temperature drop then we calculate a change in enthalpy (heat content) in a prescribed time.<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>Please report your results here.<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>Thanks<br>Crispin<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p></div></body></html>