Carlos A. C. Altemani

Convective cooling of three discrete heat sources in channel flow

A numerical investigation was performed to evaluate distinct convective heat transfer coefficients for three discrete strip heat sources flush mounted to a wall of a parallel plates channel. Uniform heat flux was considered along each heat source, but the remaining channel surfaces were assumed adiabatic. A laminar airflow with constant properties was forced into the channel considering either developed flow or a uniform velocity at the channel entrance. The conservation equations were solved using the finite volumes method together with the SIMPLE algorithm. The convective coefficients were evaluated considering three possibilities for the reference temperature. The first was the fluid entrance temperature into the channel, the second was the flow mixed mean temperature just upstream any heat source, and the third option employed the adiabatic wall temperature concept. It is shown that the last alternative gives rise to an invariant descriptor, the adiabatic heat transfer coefficient, which depends solely on the flow and the geometry. This is very convenient for the thermal analysis of electronic equipment, where the components’ heating is discrete and can be highly non-uniform.

Thermal Design of a Protruding Heater in Laminar Channel Flow

The conjugate heat transfer from a single block heater mounted on a conductive wall of a parallel plates channel was investigated under conditions of laminar airflow. The heater cooling was attained by direct forced convection to the airflow and by conduction through its contact with the channel wall. The investigation was performed for a two-dimensional configuration with fixed channel geometry and variable heater height, where the heater upstream edge was centered on the channel wall. At the channel entrance the flow velocity and temperature were uniform. The channel wall thickness was constant and its thermal conductivity was varied in the range from 0 to 80 that of the air, while the heater block thermal conductivity was equal to 500 that of the air. The conservation equations were solved numerically by the control volumes method with the SIMPLE algorithm. The results were expressed in dimensionless form, considering three distinct constraints: fixed flow rate, fixed channel flow pressure drop and fixed pumping power. The last two constraints present an optimum heater height, but care was taken to avoid that the flow recirculation downstream the heater extended beyond the channel length. The heat transfer enhancement promoted by wall conduction was clearly indicated.Copyright

Optimum Fins Spacing and Thickness of a Finned Heat Exchanger Plate

The thermal design of a counterflow heat exchanger using air as the working fluid was performed with two distinct goals: minimum inlet temperature difference and minimum number of entropy generation units. The heat exchanger was constituted by a double-finned conductive plate closed by adiabatic walls at the fin tips on both sides. The hot and cold air flows were considered in the turbulent regime, driven by a constant pressure head. The thermal load was constant, and an optimization was performed to obtain the optimum fin spacing and thickness, according to the two design criteria. A computer program was employed to evaluate the optimum conditions based on correlations from the literature. The results obtained from both design criteria were compared to each other. A scale analysis was performed considering the first design goal and the corresponding dimensionless parameters were compared with the results from the correlations.

Conjugate Cooling of a Discrete Heater in Laminar Channel Flow

Electronic components are usually assembled on printed circuit boards cooled by forced airflow. When the spacing between the boards is small, there is no room to employ a heat sink on critical components. Under these conditions, the components’ thermal control may depend on the conductive path from the heater to the board in addition to the direct convective heat transfer to the airflow.The conjugate forced convection-conduction heat transfer from a two-dimensional strip heater flush mounted to a finite thickness wall of a parallel plates channel cooled by a laminar airflow was investigated numerically. A uniform heat flux was generated along the strip heater surface. Under steady state conditions, a fraction of the heat generation was transferred by direct convection to the airflow in the channel and the remaining fraction was transferred by conduction to the channel wall. The lower surface of the channel wall was adiabatic, so that the heat conducted from the heater to the plate eventually returned to the airflow. A portion of it returned upstream of the heater, preheating the airflow before it reached the heater surface. Due to this, it was convenient to treat the direct convection from the heater surface to the airflow by the adiabatic heat transfer coefficient. The flow was developed from the channel entrance, with constant properties.The conjugate problem was solved numerically within a single solution domain comprising both the airflow region and the solid wall of the channel. The results were obtained for the channel flow Reynolds number ranging from about 600 to 1900, corresponding to average airflow velocities from 0.5 m/s to 1.5 m/s. The effects of the solid wall to air thermal conductivities ratio were investigated in the range from 10 to 80, typical of circuit board materials. The wall thickness influence was verified from 1 mm to 5 mm. The results indicated that within these ranges, the conductive substrate wall provided a substantial enhancement of the heat transfer from the heater, accomplished by an increase of its average adiabatic surface temperature.

Experimental Evaluation of the Conjugate Cooling of a Protruding Heater in a Duct

Experiments were performed to investigate the conjugate forced convection–conduction cooling of a protruding heater mounted on the lower wall (substrate plate) of a rectangular duct. The heater was an aluminum rectangular block heated by means of electric power dissipation in an embedded resistance. Airflow was forced in the duct with a hydraulic diameter Reynolds number in the range from 2,000 to 6,000. Effects of the substrate plate thermal conductivity on the heater conjugate cooling were obtained from measurements with two distinct substrates: a Plexiglas plate and an aluminum plate. The experimental results for the heater conjugate cooling were described by a single dimensionless conjugate coefficient expressed as a function of the Reynolds number. The results also indicated that the conjugate coefficient is invariant with the heater power dissipation. In addition, the heater direct convective loss to the airflow was also evaluated and described by another invariant descriptor, the adiabatic Nusselt number, as a function of the duct Reynolds number.

An Invariant Descriptor for the Conjugate Cooling of Discrete Heaters in a Duct

Forced convection cooling of electronic components mounted on circuit boards may be conveniently enhanced by conductive boards because they act as heat spreaders for the components’ cooling. This gives rise to a conjugate forced convection-conduction cooling of discrete components mounted on a board. An experimental investigation was performed to show that the temperatures of two thermal mockups mounted on the lower conductive wall of a rectangular duct cooled by forced airflow may be conveniently predicted by means of dimensionless conjugate coefficients g+ij. They are invariant with the heaters power dissipation and they may be grouped in a square matrix G+. The results were expressed as functions of the airflow Reynolds number, based on the duct hydraulic diameter, in the range from 1,600 to 6,500. The conjugate coefficients were conveniently obtained from tests with a single active heater at a time. Additional tests showed that for arbitrary power dissipation in both heaters, their temperatures were well predicted by the invariant conjugate coefficients. Numerical CFD simulations were also performed for conditions similar to those of the tests and the results were compared to those of the experiments.Copyright

Nitrogen charge temperature prediction in a gas lift valve

The operation of a class of retrievable gas-lift valves (GLV) is controlled by the axial movement of a bellows. One force acting on the bellows is due to the pressure exerted by the nitrogen gas contained in the GLV dome. It depends on the nitrogen temperature, which is influenced by both the production fluid and the injection gas temperatures in the well. This work investigated this dependence for a GLV installed in a side pocket mandrel tube. Three independent procedures were used for this purpose, comprising a compact thermal model, an experimental investigation with a thermal mockup and a numerical analysis. From these, a correlation for the nitrogen temperature was proposed, based on the local production fluid and injection gas temperatures, and on their convective coefficients with the mandrel tube surfaces.