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Subjecting biocrystals to x-ray beams is used in determining the crystal structure. The exposure to x-rays results in a non-uniform heat deposition in the biocrystals that can cause thermal damage. The crystals, therefore, are convectively cooled in a cryogenic stream of N 2 or He gases to help alleviate this problem. Here all previous x-ray beam heating studies of protein crystals with 3rd generation synchrotron sources are reviewed in terms of the important modeling considerations. Thermal transport of the deposited energy in the crystal samples involve coupled internal heat conduction and external convective heat transfer phenomena. External convection is the much more difficult process to treat but reliable information here is essential to make accurate thermal predictions of the crystal temperature.Next, the latest and most accurate thermal modeling of x-ray heating of a biocrystal is presented using advanced numerical Computational Fluid Dynamics (CFD) technique. Here, the convective heat transfer is much more accurately determined than before by solving the coupled differential momentum and thermal energy equations for fluid flow and convective heat transfer around the crystal. Average h̅, as well as local values of the convective heat transfer coefficients, hθ, are obtained. The detailed internal temperature distribution inside the crystal is also simultaneously determined, by taking into account the spatial variation in heat absorption and in the local rate of convective heat transfer around the crystal caused by the complex 3-D external fluid flow field. Also, the effect of the important flow and heat transfer parameters such as: gas velocity and gas properties, crystal size and thermal conductivity, and incident beam conditions (intensity and beam size) are all illustrated with comparative examples.This source term is then generalized by selecting a range between the limiting cases of near uniform heat generation throughout the sphere to localized surface “laser” heating. The diameter of the sphere is increased from 0.2 mm to 2 mm. Also, three different values of k (W/mK) are considered which are representative of high (silver), moderate (water) and low (gas) thermal conductivities. The key results reported in this numerical study are: the maximum internal temperature, the local and average Nusselt numbers with comparison to the solutions for isothermal wall boundary condition case. Finally, the diameter of the source beam striking the sphere is varied (small and large) without altering the incident power, which results in significant change in the magnitude of the maximum temperature and an increased variation in the internal local temperature distribution. |
