A Report on Some Heat Transfer Experiments

Dale Andreatta dandreatta@SEAlimited.com November 14, 2004, Updated January 17, 2005

Executive Summary

In the latter part of 2004 a series of about 100 tests was conducted relative to increasing the heat transfer efficiency of cookstoves. The goals were to come up with a simple way of virtually guaranteeing high efficiency in a cookstove, or, failing that goal, to learn about what conditions make a cookstove efficient. The first goal was not achieved, and this report covers what was learned relative to the second goal.

Almost all tests were done using a simulated wood fire with natural gas as the fuel. The
gas was burned in a low velocity fully non-premixed manner, and the flame appeared to
be similar to a wood fire. The firepower could be precisely controlled and measured by
varying and measuring the gas flowrate. Under conditions of limited air considerable
soot was formed, much like a wood flame. Most tests were done at one of 3 levels of
firepower. This approach appears to be very good for studying the heat transfer process,
producing repeatable results in a relatively short time.

A new technique was also developed for data reduction, using easily measured variables
to estimate parameters that are important in assessing the cookstove performance. The
mass flow rate through the stove, average gas temperature coming up through the riser,
air-fuel ratio, and log-mean temperature difference at the cook pot were estimated using
this technique. All of these variables would be difficult to directly measure, and all are important in understanding the workings of a cookstove.

Some preliminary work was done measuring temperature distributions in the gas around
the pot. The temperatures and temperature gradients near the pot are much higher on the
bottom of the pot than on the sides. Some preliminary studies were also made of
unconfined flames, more or less simulating a 3-stone fire. While unconfined flames can
lead to very high heat transfer and efficiency, the conditions which produce high
efficiency tend to be the conditions produce a lot of soot.

Out of the work with temperature measurements, a hypothesis was developed regarding
what I call the self-stratifying effect of the gases under the pot. The gases under the pot seem to segregate themselves automatically, with the hottest gases around 3 mm from the pot bottom. Evidence for this hypothesis is presented. If the self-stratifying effect is real, it could lead to a re-thinking of how best to optimize heat transfer.

In the more conventional stoves the most important variable by far in determining the
heat transfer to the pot is the temperature of the gases coming up through the riser.
Within a fairly narrow band of scatter, the heat transfer to the pot is a linear function of this temperature. Another way to correlate this data is to calculate a log-mean temperature difference, which is loosely defined as the effective average difference in temperature between the pot and the gases that are flowing next to the pot. The heat transfer is proportional to this log mean temperature difference.

The corollary to this finding is that changes to the stove are generally ineffective except in how they affect the average riser temperature. For example, the common belief is that tight skirts and tight flow passages increase heat transfer by forcing hot air against the sides of the pot or the bottom of the griddle in a griddle (plancha) stove. My conclusion is that this is mostly false, that skirts and tight passages may be helpful, but mostly because they choke off the excess air flowing through the riser and keep the average riser temperature higher. The bases for this conclusion are laid out in detail. A short list of principles for getting good efficiency is included.

Conditions which lead to high efficiency (relatively low ratio of air to fuel) are the conditions than can lead to higher pollution, hence the air-fuel rati needs to be controlled within a tight band to give good efficiency without high pollution. In all cases tested, the air-fuel ratio was much higher than that required for combustion, suggesting there is still room to decrease the air to fuel ratio further, perhaps by affecting the mixing.

A number of schemes were tried for increasing the heat transfer regardless of the combustion conditions. None of these techniques were successful, but are reviewed here.

It is generally believed that bigger pots lead to better heat transfer because more area is available for heat transfer. This document contains a summary of a brief set of tests that was performed to test this theory. The basic conclusion was that bigger pots do lead to
increased heat transfer, however the gains are not large.

Throughout this report a number of “action items” are noted, ideas or experiments that would be good to try.

For more detail please see the attached report (in pdf):