Characteristic length and cooling circle
Modular design, outsourcing and open innovation create a complex network of possible end product embodiments in product design for the consumer market. Analyzing each and every possibility in detail is out of the question as resources are limited and module details are often proprietary information. To meet this challenge the thermal engineer needs a feeling about what will and what will not work out, without getting bogged down in too much detail. This paper presents the concepts of characteristic length and cooling circle as an approximate measure for the extent of heat spreading in beam and in flat thin plate geometries. Beam and flat thin plate geometries are widely used by mechanical designers, and being able to assess how much of the geometry is actually thermally affected by a heat source is of great value in thermal decision making and communication. For both geometries an application is discussed and compared to CFD simulations to assess the validity of the approach.
Heat spreading from a small source on a thin plate
In view of the trend towards higher power densities in ever smaller package envelopes, heat spreading of a small source on a thin plate is gaining in importance. In this paper, an alternative description is presented to the popular Song-Lee-Au (SLA) approach. A length scale is derived, depending on geometry, heat transfer coefficient, and material parameters, and the relevance of this length scale is investigated. It is shown that the length scale has physical significance both for the distance that heat spreads out over the plate and for the total amount of heat cooled away. The work is the extension of this author's earlier work on Characteristic Length and Cooling Circle to the domain of finite thin plates and small sources. Thermal resistances were calculated with the Cooling Circle and with SLA, and the results were compared to numerical simulations. In the numerical comparison the difference between the two approaches emerged as different assumptions as to the thermal conductivity of the heat source. Cooling Circle assumes a thin plate, and a uniform temperature, thus very large thermal conductivity, in the source area. SLA incorporates the effect of spreading in the plate thickness, and for the thin plates comparison this boils down to the use of baseplate conductivity in the source area. Both methods match well with the numerical results. For larger plates, the Cooling Circle approach continues to match well with the numerical values. SLA values for large thin plates resulted in higher resistances, contrary to both numerical results and physics based expectations. This was attributed to the SLA correlations being used outside the range for which they were validated. The Cooling Circle approach has great engineering significance as it enables quick engineering estimations and provides a helpful mental image.
Percussion marks on bone surfaces as a new diagnostic of hominid behaviour
Since stone tool cut marks and carnivore tooth marks were first described systematically on Plio-Pleistocene archaeological bone1,2, bone surface modification has played a prominent role in interpreting early archaeological site formation and hominid behaviour3–15. Here we introduce a new class of bone surface modification, which we call percussion marks. Percussion marks were produced during experimental breakage of marrow bones, and occur as pits or grooves impressed on a bone's surface by natural protrusions on the granitic hammerstone and anvil used. Although percussion marks can superficially mimic carnivore tooth marks, they nonetheless are closely associated with hammerstone impact notches and show consistent micromorphological features which distinguish them from tooth marks and other classes of bone surface modification. Given indications of prehistoric hammer-stone breakage of marrow bones1,7, an awareness of percussion marks is critical for accurately identifying the biological agents of bone modification at archaeological sites and provides a new diagnostic of carcass processing by hominids.