The effects of flow maldistribution caused by partial blockage of the inlet of a flat rectangular duct were studied experimentally. Local heat transfer coefficients were measured on the principal walls of the duct for two blockages and for Reynolds numbers spanning the range between 6000 and 30,000. Measurements were also made of the pressure distribution along the duct, and the fluid flow pattern was visualized by the oil-lampblack technique. Large spanwise nonuniformities of the local heat transfer coefficient were induced by the maldistributed flow. These nonuniformities persisted to far downstream locations, especially in the presence of severe inlet flow maldistributions. Spanwise-average heat transfer coefficients, evaluated from the local data, were found to be enhanced in the downstream portion of the duct due to the flow maldistribution. However, at more upstream locations, where the entering flow reattached to the duct wall following its separation at the sharp-edged inlet, the average coefficients were reduced by the presence of the maldistribution.

Heat transfer experiments were performed for a high-aspect-ratio (∼18) rectangular duct having a sharp-edged inlet, with air being drawn into the inlet from a large upstream space. The experiments encompassed data runs where both of the principal walls of the duct were isothermal (at the same temperature) and other runs where one wall was isothermal while the other was adiabatic. Local heat transfer coefficients were determined for all runs. It was found that flow separation at the duct inlet played a decisive role in shaping the axial distribution of the heat transfer coefficient in the thermal entrance region. Of particular note is a high heat transfer peak at the point of flow reattachment. The peak is situated at an axial station less than one hydraulic diameter from the inlet and moves upstream with increasing Reynolds number. The heat transfer coefficients for symmetric and asymmetric heating are identical in the initial portion of the thermal entrance region. Deviations occur farther downstream but do not exceed more than about 7 percent. The entrance length for asymmetric heating is significantly greater than that for symmetric heating.

Technical Briefs Heat Transfer in a Tube Downstream of a Tee in which Airstreams of Different Temperature are Mixed E. M. Sparrow, E. M. Sparrow Department of Mechanical Engineering, University of Minnesota, Minneapolis, Minn. 55455 Search for other works by this author on: This Site PubMed Google Scholar N. Cur, N. Cur Department of Mechanical Engineering, University of Minnesota, Minneapolis, Minn. 55455 Search for other works by this author on: This Site PubMed Google Scholar R. G. Kemink R. G. Kemink Department of Mechanical Engineering, University of Minnesota, Minneapolis, Minn. 55455 Search for other works by this author on: This Site PubMed Google Scholar Author and Article Information E. M. Sparrow Department of Mechanical Engineering, University of Minnesota, Minneapolis, Minn. 55455 N. Cur Department of Mechanical Engineering, University of Minnesota, Minneapolis, Minn. 55455 R. G. Kemink Department of Mechanical Engineering, University of Minnesota, Minneapolis, Minn. 55455 J. Heat Transfer. Aug 1980, 102(3): 568-570 (3 pages) https://doi.org/10.1115/1.3244342 Published Online: August 1, 1980 Article history Received: January 4, 1980 Online: October 20, 2009

The heat transfer and pressure drop characteristics for an array of colinear, equally spaced plates aligned parallel to the flow in a flat rectangular duct have been studied experimentally. The periodic interruptions (i.e., the gaps between the plates) preclude the attainment of hydrodynamic and thermal development of the type that is encountered in conventional duct flows, but a periodic fully developed regime can exist. Measurements of the heat transfer coefficients for the successive plates of the array affirmed the periodically developed regime and demonstrated the developmental pattern leading to its attainment. The thickness of the plates in the array was varied parametrically. In general, the Nusselt number increases with plate thickness. Thickness-related increases in the fully developed Nusselt number of up to 65 percent were encountered. The presence of the interruptions serves to augment the heat transfer coefficients. In the fully turbulent regime, the heat transfer coefficients are on the order of twice those for a conventional duct flow. The pressure drop also increases with the plate thickness.

Experiments have been performed to determine the heat-transfer characteristics for each plate of a two-plate colinear array aligned parallel to the flow direction and situated in an airflow in a flat rectangular duct. The pressure drop associated with the plates was also determined. The plate thickness and the interplate spacing were parametrically varied (for a fixed plate length), as was the Reynolds number. It was found that while the Nusselt number may either increase or decrease with plate thickness, depending on the operating conditions, it should increase for conditions corresponding to those commonly encountered in an interrupted-plate heat exchanger. The pressure drop increase caused by increasing plate thickness is greater than the largest thickness-related increase of the Nusselt number. The presence of the inter-plate gap affects the Nusselt numbers for both the first and second plates, but to a greater extent for the latter. In many cases, the curves of Nusselt number vs spacing attain a local maximum and indicate that the conventional spacing-to-length ratio of unity is not necessarily optimal.

Thermal conductivity measurements were performed to determine the characteristics of hollow glass microspheres as an insulating material and as an opacifying agent for other insulations. The experiments were carried out with a radial flow heat transfer apparatus especially designed to suppress extraneous heat transfers, both internal and external to the heated section, and to provide uniform temperatures on the bounding surfaces. Three types of microsphere insulations were investigated, differing in bulk density and in the presence or absence of an aluminizing coating. The thermal conductivity of the microsphere insulations was found to be about one and a half times that of stagnant air over a wide temperature range. Additional experiments, involving the use of an opacifier (powdered silicon), demonstrated that radiative transfer has a minor effect on the thermal conductivity of microsphere insulations. This finding was corroborated by the fact that the high-temperature conductivity of the aluminized microspheres was not appreciably different from that of the uncoated microspheres. Another set of experiments was performed in which microsphere insulation was added to opacify silica aerogel, a fine powder insulation that is markedly affected by radiative transfer. The presence of the microspheres brought about reductions in conductivity of almost a factor of two at an optimum mixture ratio of the constituents. Furthermore, it was found that the conductivity of such a mixture was lower than that of either constituent, thereby illustrating their synergistic interaction.