EXPERIMENTAL INVESTIGATION OF MIXED CONVECTION HEAT TRANSFER CAUSED BY FORCED-JETS IN LARGE ENCLOSURE

ABSTRACT This research investigates experimentally mixed convection and heat transfer augmentation by forced jets in a large enclosure, at conditions simulating those of actual passive containment cooling systems and scales approaching those of actual containment buildings or compartments. The experiment was designed to measure the key parameters governing the heat transfer augmentation by forced jets and investigate the effects of geometric factors, including the jet diameter, jet injection orientation, interior structures, and enclosure aspect ratio. The tests cover a variety of injection modes leading to flow configurations of interest that contribute to reveal the nature of mixing and stratification phenomena in the containment under accident conditions of interest. By nondimensionalizing the governing equations, the heat transfer of mixed convection can be predicted to be controlled by jet Archimedes number and geometric factors. Using a combining rule for mixed convection and appropriate forced and natural convection models, the correlations of heat transfer augmentation by forced jets are developed and then tested by experimental data. The effects of jet diameter, injection orientation, interior structures, and enclosure aspect ratio on heat transfer augmentation are illustrated with analysis of experimental results

Shox2 is essential for the differentiation of cardiac pacemaker cells by repressing Nkx2-5

The pacemaker is composed of specialized cardiomyocytes located within the sinoatrial node (SAN), and is responsible for originating and regulating the heart beat. Recent advances towards understanding the SAN development have been made on the genetic control and gene interaction within this structure. Here we report that the Shox2 homeodomain transcription factor is restrictedly expressed in the sinus venosus region including the SAN and the sinus valves during embryonic heart development. Shox2 null mutation results in embryonic lethality due to cardiovascular defects, including an abnormal low heart beat rate (bradycardia) and severely hypoplastic SAN and sinus valves attributed to a significantly decreased level of cell proliferation. Genetically, the lack of Tbx3 and Hcn4 expression, along with ectopic activation of Nppa, Cx40, and Nkx2-5 in the Shox2−/− SAN region, indicates a failure in SAN differentiation. Furthermore, Shox2 overexpression in Xenopus embryos results in extensive repression of Nkx2-5 in the developing heart, leading to a reduced cardiac field and aberrant heart formation. Reporter gene expression assays provide additional evidence for the repression of Nkx2-5 promoter activity by Shox2. Taken together our results demonstrate that Shox2 plays an essential role in the SAN and pacemaker development by controlling a genetic cascade through the repression of Nkx2-5

Development of C-SiC ceramic compact plate heat exchangers for high temperature heat transfer applications

This paper investigates the use of polymer and liquid silicon infiltrated carbon/silicon-carbide composite (C-SiC) materials for the development of inexpensive compact heat exchangers, as part of efforts for thermochemical hydrogen production. These heat exchangers will be capable of operating in the temperature range of 500 to 1400°C with high-pressure helium, liquid fluoride salts (a potential intermediate heat transfer fluid), or other corrosive gases such as SO3 and HI. C-SiC composites have several potentially attractive features, including ability to maintain nearly full mechanical strength to temperatures approaching 1400°C, inexpensive and commercially available fabrication materials, and the capability for simple forming, machining and joining of carbon-carbon performs, allowing the fabrication of highly complex component geometries. To meet cost goal, candidate materials must have relatively low bulk costs, and fabrication methods must extrapolate to low-cost mass manufacturing. Composite compact offset fin plate heat exchangers concept has been developed to meet the above functional and cost goals, which will serve as the intermediate heat exchanger (IHX) to transfer high temperature heat from a helium-cooled high temperature nuclear reactor to a liquid salt intermediate loop which couples to hydrogen production loops. The IHX uses offset fin structures with fin width and height at 1 mm scale. The detailed local and global thermal mechanical stress analyses show that the designed composite plate heat exchanger can tolerate pressure difference up to 9 MPa and large temperature difference from two fluid sides. Two potential low cost methods to fabricate C-SiC are liquid silicon (melt) infiltration (MI) and Polymer Infiltration and Pyrolization (PIP). Mechanical strength tests on MI coupons show above 200 MPa failure stress. Leak-tight pyrolytic carbon coatings have been successfully applied on MI C-SiC coupons and excellent helium hermeticity were obtained under high pressure and stress after coating. PIP plates with high-quality millimeter-scale fins formed using teflon molds have been successfully demonstrated. The teflon molds were proven to be reusable, so that the process could be extrapolated to inexpensive mass fabrication of compact ceramic heat exchangers. Prototype test heat exchangers are being fabricated basing on both MI and PIP methods