# Comparison of the Axial Fan and Synthetic Jet Cooling Systems
## 1 Introduction  
Proper thermal management is crucial in order to achieve the designed lifetime and correct functioning of electronic devices. The increasing power density of modern devices often renders the classic cooling methods insufficient. However, despite the development of such passive cooling materials and devices as heat pipes [1, 2], phase change materials [3, 4] and thermomagnetic devices [5, 6], the classic fin heat sinks are still the most common. A synthetic jet actuator (SIA) consists of a cavity with at least one orifice and an element capable of reciprocating movements, such as a loudspeaker, piezoelectric diaphragm or mechanical piston. In order to generate a synthetic jet flow, this movable or deformable element named the actuator is embedded into one of the walls of the cavity. By theperiodical change of the cavity volume, a synthetic jet (SJ) is generated [7]. In recent years SJAs have found a lot of applications, such as flow control, mixing or heat transfer enhancement [8]. The investigations of the heat transfer enhancement with the synthetic jet usually concern one of the two configurations: flat plate impingement cooling and heat sink cooling. [PERSON] and [PERSON] [9] presented an experimental investigation of the synthetic jet cooling mechanism based on the jet impinging onto the heated surface. The synthetic jet and continuous jet impingement cooling performance as a function of the dimensionless jet to surface distance were compared at the same Reynolds number of 445. The synthetic jets were about three times more effective in cooling than continuous jets at the same Reynolds number. [PERSON] et al. [10] presented the heat transfer characteristic of a synthetic jet impingement cooling. The experiments were performed for a wide range of input parameters: the Reynolds number was in the range of 1500-4200, and the dimensionless jet to surface distance was in the range of 0-25. A direct comparison between the continuous axisymmetric jet and the synthetic jet for the same set of conditions was performed. The behavior of the average Nusselt number with synthetic jet cooling was found to be similar to that obtained for a continuous jet. The maximum Nusselt number was about 10% higher for the continuous jet compared to the synthetic jet at a Reynolds number of the order of 4000. [PERSON] and [PERSON] [11] performed an experimental investigation on the heat sink cooling performance with the use of a synthetic jet for various sets of geometrical configurations. The authors also presented a direct comparison between active cooling methods with a fan and synthetic jet at the same input power levels. The heat sink's thermal resistance with the synthetic jet cooling strongly decreased with the increased input power, while for the fan-cooled heat sink, the thermal resistance above 1 W was found to be almost constant. The heat sink's thermal resistance with a synthetic jet actuator and a fan at the same input power of 1.35 W was 0.90 and 0.54 K/W, respectively, which means that the fan's cooling performance was 67% higher than the performance of the synthetic jet's cooling. With the input power increased to 5.5 W, the heat sink thermal resistance was comparable for both cooling systems, but the fan-cooled heat sink still overperformed synthetic jet cooling by 6%. [PERSON] et al. [12] investigated the impact of the geometric parameters (the number of fins, the fin length and the fin height) on the heat flux of the radial heat sink under natural convection conditions. They showed that the optimal values for the fin number and fin length exist that result in the lowest effective heat sink temperature. In [13], the same group of researchers simultaneously optimized the thermal performance and mass of the three types of radial heat sinks. They showed that the optimization of mass deteriorates the values of the thermal parameters and vice versa; therefore, they used the Pareto optima to indicate the overall best solution. Similar optimization was conducted for the pin-fin radial heat sink by [PERSON] et al.