(145c) Unsteady State Heat Transfer Modeling in the Undergraduate Engineering Laboratory to Estimate Time of Death

Historically, estimation of time of death has always been a valuable forensic tool. In England, 925 AD, St. John of Beverly was appointed ?keeper of the pleaser of the crown?, later to be shortened to ?crowner?, then finally to ?coroner?. At that time, only crude ?forensic? tools of rigor mortis and corpse warmth were used to gage time of death. It was not until 1600, that the first criminal autopsy was conducted by Ambrose Pare (French physician) [1,2].

By 1811, an English physician named John Davey had formulated his famous law based upon the principle of algor mortis (slow cooling of a warm-blooded corpse). Davey's Law states that 1.5 oF is lost per hour as measured in the armpit. Therefore, at least in theory, it should be possible to record the temperature of a corpse and having knowledge of the heat loss rate curve, be able to estimate time of death. In reality, the rate curve is complicated by many factors and it is often difficult to pinpoint time of death.

Over the years, there have been numerous models proposed to estimate time of death [3]. Results of these models can be useful to crime scene investigators, but are not admissible into courts of law. There is too much error and variability associated with these current models.

Students in the undergraduate engineering laboratory explored use of an unsteady state heat transfer model to estimate time of death. A large diameter wood cylinder was constructed to approximate the trunk of a human body. Interestingly enough, wood has approximately the same thermal diffusivity as the human body. A hole was drilled and a thermocouple was placed at the approximate relative location of the anus. The cylinder was heated to 37 deg C (98.6 deg F) in an oven. At time zero, the cylinder was removed from the oven and experimental data of temperature decay was collected until the cylinder cooled to room temperature (about 21 deg C).

As a laboratory exercise, students are told a crime scene investigator (CSI) had examined an apparent murder victim. The CSI had recorded the victim's rectal temperature between a two hour interval. Also, noted was the ambient temperature within the room. Students are asked to estimate time of death to aid in solving this crime. Students use a Fourier series [4] to model experimental data collected from unsteady state heat transfer from the wooden cylinder. They calibrate their model and test the sensitivity of the model to variations in crime scene conditions. Based upon their test results, students are able to estimate time of death.

1. Sachs, Jessica S., Corpse: Nature, Forensics, and the Struggle to Pinpoint Time of Death, Perseus Books Group: Basic Books, NY, October 2002.

2. Miller, Hugh, What the Corpse Revealed: Murder and the Science of Forensic Detection, St. Martin's Press, NY, June 1999.

3. Henssge, Claus, The Estimation of the Time Since Death in the Early Postmortem Period, 2nd Ed., London: Arnold Publishing, 2002.

4. Incropera, F.P. and D.P. DeWitt, Fundamentals of Heat and Mass Transfer, J. Wiley & Sons, New York, 2002.