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CHAPTER 1
INTRODUCTION
1.1 BACKGROUND
The world has been witnessing hectic growth in the field of electronics. Added to this is the rapid development in the miniaturization of electronic components and ever-increasing heat flux densities. All of these changes have opened new vistas and created new challenges for researchers working on thermal management solutions. Heat is an inevitable by-product of every electronic device, and unless dissipated, it causes buildup of temperatures, resulting in decreased performance and poor reliability. The reliability of a device or equipment is a direct measure of the possible frequency of failure as a function of time. Many factors affect the reliability of electronic components. These include temperature, humidity, vibration, and shock, among other things. In several applications like laptops, mobile phones, digital cameras, and electronics in spacecraft, the thermal management solution needs to be reliable, safe, and energy efficient. Cost is another critical concern, though it is not the major one in an application like cooling of electronic components in spacecrafts. Growing flux densities, harsher environments, miniaturization, and ever-increasing performance expectations continue to challenge a thermal engineer.
Thus, the aforementioned growth in electronics demands innovative solutions in thermal management. The absolute power levels used in microelectronic devices range from a few tens to a few hundreds of watts. Even so, heat fluxes can be significant, varying from 5 W/cm2 on a printed wiring board to 2,000 W/cm2 for a semiconductor laser. Providing cooling solutions for high heat flux requires considerable novelty. The cost also needs to be quite competitive. A bird’s-eye view of the history of thermal management solutions is presented in Figure 1.1. The exponential curve shows the rapid increase in the heat flux and the changes that have come about in cooling technologies. Future cooling solutions around multiphase heat transfer technologies, among other alternatives, are being developed.
Figure 1.1. Thermal management scenario
Source: IBM, USA
1.2 POSSIBLE COOLING STRATEGIES
Thermal management solutions can be broadly classified as follows: a) active cooling techniques and b) passive cooling techniques. The term active cooling suggests that the cooling is assisted with the following devices: a pump, a blower, or a compressor. In view of this, it is apparent that active cooling techniques can help with handling high fluxes, and it is possible to bring down temperatures below the ambient level with the help of these new technologies. Jet impingement technology uses air or any other liquid; forced convection, spray cooling, and refrigeration systems are also effective active cooling solutions. However, the use of these technologies involves higher operating costs and increased size and weight of the equipment. Passive cooling subsystems do not require external power. Some of the passive cooling systems commonly used include natural convection, use of phase change materials, and thermosiphons. Passive cooling techniques are generally used for lower heat-flux densities. In what follows, we will be discussing some of the active and passive cooling techniques commonly used.
1.2.1 ACTIVE COOLING
Forced convection with air: Here, the electronic components are cooled by circulating air with a fan. The heat transfer coefficients associated with the use of air are very low. Therefore, in order to cool high heat fluxes, the air velocity required may be very high, leading to a need for higher pumping power. The noise level will also increase, as the fan speed increases. In addition, use of fans increases the possibility of mechanical failure. In consideration of all these shortcomings, applications that use forced convection with air are rather limited in use.
Forced convection with liquid: For cooling high heat flux electronic components like computer microprocessor and power electronics, liquid-cooled plates and liquid cooling are a good choice. Liquid cooling involves circulating a coolant (water or refrigerant/water mixtures) through a cold plate. This plate absorbs the heat from heat-generating electronic components and then transfers it with a liquid-to-air heat exchanger or liquid-to-liquid heat exchanger to reduce and bring it on par with ambient temperature. This process is repeated and the cooled liquid once again becomes ready to absorb the heat.
Spray cooling: Spray cooling involves dispersing fine droplets onto a heated surface. Here, large amounts of heat can be removed as the droplets evaporate instantly due to latent heat of evaporation. Very high heat transfer rates can be obtained in boiling with sprays, for example, from a pool. Spray cooling can be applied for large surfaces, and the impact of droplets is usually negligible to cause any erosion. However, parts like filters, pumps, and auxiliary equipment add to cost and weight.
Jet impingement cooling: Here, a cold jet of fluid impinges on the target surface. This mechanism usually has a high-velocity jet with its nozzle emanates from a hole or slot or an array of holes/slots. The impingement results in very high heat transfer rates. Other cooling techniques like Peltier cooling or thermoelectric cooling have also been developed.
1.2.2 PASSIVE COOLING
Natural convection: Natural convection is used in situations where heat transfer occurs because of the differences in fluid density caused by temperature or concentration gradients. The heated fluid becomes less dense and rises up, and the surrounding fluid rushes in to the replace the heated fluid. This is useful for standalone applications such as modems and small computers having an array of printed circuit boards (PCBs) mounted within an enclosure. The heat transfer rates in natural convection cooling are limited to small heat fluxes. Nevertheless, it is noiseless and highly reliable.
Thermosiphons: Thermosiphons operate much in the same way as a heat pipe does. The key difference, though, is that there is no capillary structure present as would be seen in heat pipes. In view of this, the evaporator needs to be fixed vertically below the condenser, which ensures that the condensate returns to the evaporator with the aid of gravity. Thermosiphons are used in heat pumps, water heaters, and boilers. Although thermosiphons have limitations like orientation and capacity, often they are a good alternative for mechanical pumps.
Heat pipes: A heat pipe is a device that essentially consists of an evaporator and condenser that are separated by an adiabatic section. The working fluid is converted into vapor where heat is absorbed by the evaporator. The vapor dissipates heat generated by the condenser and becomes a liquid once again. A capillary wick is then used to bring the liquid condensate back to the evaporator. The thermal performance of the heat pipe is a strong function of the wick, working fluid, and the size and shape of the heat sink and is a weak function of orientation.
Phase change material (PCM)-based cooling: PCMs are highly effective heat storage materials that undergo a phase change at a certain temperature known as the phase change temperature. Here, the material latent heat is absorbed at the place where electronics dissipate the heat. When the latent heat is released, the PCM once again gets solidified.
The melting and solidification are nearly isothermal. Thermal management systems using PCM-based heat sinks have been used in a wide range of applications, such as spacecraft and avionics thermal control, personal digital assistants, iPods, mobile phones, digital cameras, and notebooks. They can be used especially in situations where the heat dissipation is intermittent or periodic as is the case in most of the above devices.
1.3 ADVANTAGES OF PASSIVE COOLING TECHNIQUES OVER ACTIVE COOLING METHODS
Passive cooling technologies offer many advantages over active cooling methods. They are useful in situations where the space is a premium and active cooling devices cannot be used. Since there is no need for electricity to run these, maintenance costs are very low. Passive systems are quite efficient and cost-effective in applications like avionics.
The ever-increasing demands for better performance of electronic equipment have propelled the development of new and innovative thermal management solutions like PCM-based cooling. As the latent heat rising from the fusion of PCMs is relatively high, only a small amount of PCM is required to handle the heat storage requirements for a plethora of applications. Details regarding the classification of the PCMs and their salient features are discussed in the next section.
1.4 PHASE CHANGE MATERIALS
PCMs can be broadly classified as organic, inorganic, and eutectic. A classification of PCMs is given in Figure 1.2.
Paraffin and non-paraffin compounds come under the category of organic PCMs. Organic materials involve congruent melting, which means that melting and freezing, even if they occur repeatedly, no phase segregation is caused. Paraffin is available for use in large temperature ranges and is safe, reliable, l...
