No abstract

Micro heat pipes are small structures that will be used to cool microscale devices. They function much like their conventional counterparts, with a few exceptions, most notably the absence of a wick. It is expected that water-filled micro heat pipes will be able to dissipate heat fluxes on the order of 10-15 W/cm 1 (100,000-150,000 W/m 2 ). This work addresses the modeling of a micro heat pipe operating under steady-state conditions. A one-dimensional model of the evaporator and adiabatic sections is developed and solved numerically to yield pressure, velocity, and film thickness information along the length of the pipe. Interfacial and vapor shear stress terms have been included in the model. Convection and body force terms have also been included in the momentum equation, although numerical experiments have shown them to be negligible. Pressure, velocity, and film thickness results are presented along with the maximum heat load dependence on pipe length and width. Both simple scaling and the model results show that the maximum heat transport capability of a micro heat pipe varies with the inverse of its length and the cube of its hydraulic diameter, implying the largest, shortest pipes possible should be used.

In metal additive manufacturing (AM) processes, parts are manufactured in layers by sintering or melting metat or metal altoy powder under the effect of a powerful taser or an electron beam. As the laser/electron beam scans the powder bed, it melts the powder in successive tracks which overlap each other. This overlap, called the hatch overlap, results in a continuous cycle of rapid melting and resolidiflcation of the metat. The metting of the metal from powder to liquid and subsequent sotidification causes anisotropic shrinkage in the layers. The thermat strains caused by the thermat gradients existing between the different layers and between the layers and the substrate leads to considerabte thermat stresses in the part. As a resutt, stress gradients devetop in the different directions of the part which lead to distortion and warpage in AM parts. The deformations due to shrinkage and thermal stresses have a signiflcant effect on the dimensional inaccuracies of the final part. A three-dimensional thermomechanicat finite etement (EE) modet has been devetoped in this paper which calculates the thermat deformation in AM parts based on slice thickness, part orientation, scanning speed, and materiat properties. The EE modet has been vatidated and benchmarked with resutts atready avaitabte in titerature. The thermat deformation modet is then superimposed with a geometric virtuat manufacturing model of' the AM process to catcutate the form and runout errors in AM parts. Einalty, the errors in the criticat features of the AM parts calculated using the combined thermat deformation and geometric modet are corretated with part orientation and stice thickness.

Blood platelets are produced by large bone marrow (BM) precursor cells, megakaryocytes (MKs), which extend cytoplasmic protrusions (proplatelets) into BM sinusoids. The molecular cues that control MK polarization towards sinusoids and limit transendothelial crossing to proplatelets remain unknown. Here, we show that the small GTPases Cdc42 and RhoA act as a regulatory circuit downstream of the MK-specific mechanoreceptor GPIb to coordinate polarized transendothelial platelet biogenesis. Functional deficiency of either GPIb or Cdc42 impairs transendothelial proplatelet formation. In the absence of RhoA, increased Cdc42 activity and MK hyperpolarization triggers GPIb-dependent transmigration of entire MKs into BM sinusoids. These findings position Cdc42 (go-signal) and RhoA (stop-signal) at the centre of a molecular checkpoint downstream of GPIb that controls transendothelial platelet biogenesis. Our results may open new avenues for the treatment of platelet production disorders and help to explain the thrombocytopenia in patients with Bernard–Soulier syndrome, a bleeding disorder caused by defects in GPIb-IX-V.