Power Electronics

Hybrid Heat Sink Technology Promises Temperature Uniformity

A recently proposed hybrid jet impingement/micro-channel heat sink technology would extract high heat-flux densities and control a target object's temperature distribution.

A proposed hybrid heat sink (patent pending) developed by the Energy for Environment and Agrometeorology research group of the University of Lleida (Spain) has been studied experimentally and numerically in a configuration for single-phase cooling. The compact, high-density heat sink has a variable internal geometry that allows it to adapt its temperature profile to the specific needs of the cooled object, which can correct inappropriate temperature distributions and help reduce potential damages to the object being cooled.

Different design parameters have been analyzed in a parametric study that aims to optimize the system according to the specific needs of different applications. A study of its performance in two-phase mode is in progress.

SCIENTIFIC PRINCIPLES

The proposed device is a hybrid jet impingement/micro-channel heat sink. Fluid enters the cooler as a jet impingement through a slot (Fig. 1).

After impact with the bottom of the heat sink, the flow is divided and enters the channels, or fin areas. These zones show a non-uniform distribution of heat-exchange elements in the refrigerant flow direction (Figs. 2a and 2b) that controls the distribution of the heat-flux extraction capacity.

Consequently, it can adapt the profile temperature to the needs of the particular application. Fig. 3 shows the fins or channels that reduce pressure drop. The fluid used in the heat sink was originally water (in the experimental device), but could also be a glycol-water mixture or another fluid, depending on the application.

Pump type depends on the dimensions of the system, from nanoscale to much larger. The experimental device (Fig. 4) has three volume distributors (a central one for the inlet and two for the outlets). Fig. 5 shows the experimental heat sink.

The influence zone of the two cooling technologies can be modified by changing the internal distribution of the design elements. In each area, the characteristics of heat-flux extraction depend on the specific parameters of the cooling technology used.

The goal of this technology is to manage the ever-increasing heat-flux densities for a wide variety of technologies in many different applications (particularly in microelectronics). This must be done in two ways.

First, it is necessary for these devices to cool the elements that generate heat and reduce their temperature to an acceptable range. Second, it's necessary to improve the temperature uniformity of the cooled object. In effect, this parameter also affects the performance of electrical systems and reduces their reliability.

Heat sinks currently achieve the first objective, but they don't offer any solution to the second one. For example, micro-channels can only minimize temperature increases in the direction of the fluid flow by increasing the flow of refrigerant (which implies increased power of the circulation pump and, therefore, to the total system cost), but cannot eliminate this temperature gradient.

As a consequence, in cooling electronic devices, most research is focused on enhancing the cooling performance and improving the resistance of the devices due to thermal cycles, matching the thermal expansion coefficient of the packaging to the one of semiconductors. The proposed hybrid heat sink could improve temperature uniformity, and the problem of mechanical stress due to varying heat-expansion coefficients would disappear.

INNOVATION AND ADVANTAGE

The proposed hybrid heat sink improves the temperature uniformity of the cooling object. Further, it can even adapt its temperature profile to meet specific needs, implying an increase in the reliability of the cooled object.

The device's internal geometry can be modified at the design stage to achieve this effect. Besides this innovative feature, the proposed heat sink will offer other advantages, such as:

  • Compact design: The fact that outlet connections of the fluid can be made in the same direction as inlets (but in the opposite sense) allows heat-sink dimensions that do not exceed those of the object to be cooled
  • Relatively low pressure losses: Inside the cooler, pressure losses are lower than in micro-channels. As the price of micro-pumps depends on the pressure drop they overcome, it directly affects system cost
  • Suit all scaled dimensions: This system can be applied with different scale dimensions. Manufacture of this design is possible even in the nanometric range, due to recent developments in extraction and deposition manufacturing techniques at this scale
  • Heating: The device can also be used to heat objects with predetermined temperature profiles by circulating a warm fluid through the heat sink

POTENTIAL APPLICATIONS

The hybrid jet impingement/micro-channel heat sink is oriented to all applications requiring a highly compact solution that can extract high densities of heat flux and distribute heat from the object that is being cooled. However, it can be also used for controlled heating of the elements.

This applies in particular to microprocessor cooling (Fig. 6) as well as cooling of high-concentration photovoltaic cells. The design has broad prospects for use in this last field.

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