Investigation of Synthetic Jets Heat Transfer and Flow Field

The continuous technology improvement is causing a proliferation of electronic systems in our society. Nowadays human society is completely dependent on such electronic devices; hence, the reliability of these systems is mandatory. The failure of these systems can be caused by many factors such as high temperature, electrical overstress and material fatigue. The system failure is mostly due to the thermal overstress caused by a continuous request of intense work. Although the majority of the electronic devices have a high efficiency, a continuous use can cause an increase in temperature which can affect the device performance until the failure. Electronic components are designed to operate within a certain temperature range thus need for a cooling system. Such a cooling system has to be designed to dissipate all the thermal power to maintain a suitable temperature and guarantee electronic component safety without affecting working performance. According to Moore’s law (1965), the number of transistors on an integrated circuit doubles every one and a half year and the transistor density is expected to be very high. Moreover, the size of these transistors decreases exponentially as silicon microfabrication processes are improved. These two components lead to an increase in the heat power which has to be carried away by cooling devices in order to let the electronic systems work properly within a suitable temperature range. Conventional cooling techniques such as heat sink, fans and liquid cooling need to be continuously optimized to face this continuous request of higher heat power dissipation and miniaturization. Such needs have led researchers to focus their efforts on the design of new technology cooling devices. Synthetic jets are an innovative device with promising features in the heat dissipation field. According to Lasance and Aarts. (2008), the benefits and improvements of synthetic jets, compared with a fan, are: • Lower noise level, • Better efficiency, • Design-friendly, • Intrinsic higher reliability, • Easier miniaturisation, • Simple noise cancellation. Synthetic jets are jets with zero-net-mass-flux synthesized directly from the fluid present in the system in which the jet device is embedded (Smith and Glezer, 1998). Such a feature obviates the need for an external input piping, making them ideal for low cost and low space applications. A synthetic jet is generated by a membrane oscillation in a cavity which produces a periodic volume change and thus pressure variation. As the membrane oscillates, fluid is periodically entrained into and expelled out from the orifice. During the injection part of the cycle the flow field could be considered as one inducted by a sink, which coincides with the orifice, while during the ejection part of the cycle, a vortex ring can form near the orifice and, under certain operating conditions (Holman et al., 2005), convects away from the orifice to form a time-average jet (Smith and Glezer, 1998). Several studies have been carried out to characterize and describe the heat transfer behaviour of a classical synthetic jet (Gutmark et al., 1982, Mahalingam and Glezer 2005, Valiorgue et al., 2008, Chaudhari et al., 2010a, Persoons et al., 2011, Arik and Icoz ,2012, Greco et al. 2014). In order to improve the heat transfer performances of synthetic jets in practical applications, some recent studies are focused on the design of different configurations. For instance, Rylatt and O’Donovan (2013) decided to confine the impinging synthetic air jet asserting that, with such a configuration, cold air was drawn from a remote location into the jet flow. Chaudhari et al. (2011) realized a particular synthetic jet device by means of a centre orifice surrounded by multiple satellite orifices. Laxmikant and Chaudhari (2015) designed also synthetic jets with diamond and oval shape orifice. Luo et al. (2006) proposed a new generation of synthetic jet consisting in two cavities sharing the same piezoelectric actuator. Lasance ans Aarts (2008) and Lasance et al. (2008) replaced the two slots with two orifices and the piezoelectric with a loudspeaker creating the so-called “dipole cooler”. This double configuration was found to be advantageous because of noise reduction (Russell et al., 1999) and improvement of heat transfer performances (Lasance et al., 2008) without a characterization of its flow field behaviour. The object of this research is: • to design a similar experimental apparatus with variable jet-axes-distance; • to deeply investigate and characterize the jets free and impinging flow field; • to evaluate and explain the jets heat transfer performance; • to compare the results with the classical single synthetic jet (under the same operating conditions). Moreover, as also reported in Persoons et al. (2011): “Although information is available for free synthetic jets insufficient knowledge is available for impinging synthetic jets, which could be the focus of future research.”, thus a study on the classical single synthetic jet impinging flow field behaviour, varying the operating parameters, has also been carried out. The thesis has been divided as following: Chapter 1 reviews current literature, Chapter 2 describes the measurements techniques employed in present investigation, in Chapter 3 the experimental rigs are shown, Chapters 4 and 5 discuss the results, for the free and impinging configurations, Chapter 6 draws final conclusions and Chapter 7 opens the path for future works.