Recent Developments in Air Pumps for Thermal Management of Electronics

Based on the first law of thermodynamics, the internal energy of a stationary system will be increasing if the net energy entering the system is positive, and as a result, the temperature of the system will be getting high. An electronic component could overheat due to high power and/or poor heat dissipation, causing various problems on efficiency, reliability, performance, and so on [1–4]. Two common strategies for thermal management of an electronic component are reducing electrical work input (power consumption) and removing thermal energy out of the system. While reducing electrical work input requires substantial coding and manufacturing efforts, removing thermal energy out of the system can be achieved with the help of forced convection at relatively low costs.

Figure 1 shows the schematic of a typical thermal management strategy that uses forced convection. The heat source generates heat, while the thermal interface material fills the microcavities between the contact surfaces of the heat source and the heat spreader. A typical heat spreader used for an electronic device is a heat sink that has large surface areas and is usually made of highly thermally conductive materials (e.g., aluminum or copper for low cost purposes) such that heat can spread around effectively. Explained by Newton's cooling law, as shown in (1), the heat transfer rate is proportional to both the heat transfer coefficient and the surface area. For a typical fan-sink configuration, when the surface area of the heat sink and the heat transfer coefficient is larger, the heat removal from the heat sink to the environment can be more effective. The air pump is exactly used to provide airflow, while the heat transfer coefficient is positively dependent on the airflow power.

where $Q˙$ is heat transfer rate, $h$ is heat transfer coefficient (highly depends on the characteristics of airflow generated by the air pump), $A$ is surface area, $Ts$ is surface temperature, and $T∞$ is ambient temperature.

Air pump or fan is a power-consuming device that transfers energy to the fluid by raising the fluid's pressure, i.e., to make the fluid move. With or without moving parts, air pumps can be categorized into mechanical and nonmechanical. Because of the moving parts, mechanical pumps are able to provide strong airflow or high pressure but have problems such as wear, noise, vibration, and volume-demanding [5–7]. On the other hand, although nonmechanical pumps do not have moving parts and do not create strong airflow or high pressure as mechanical pumps do, nonmechanical pumps are quiet and flexible in geometry, configuration, and size.

As shown in Fig. 2, this paper reviews five types of air pumps that have been using widely in or have a great potential in electronics, including four mechanical pumps (two rotary and two reciprocating) and one nonmechanical pump (electrohydrodynamic). For every type of air pump, this paper introduces the working principle first and followed by the recent developments that include the pump itself (design, characteristics, etc.) and the applications in thermal management (placement, integration, etc.). In the end, this paper uses a radar chart to compare the reviewed air pumps in terms of the flow rate, pressure, noise, flexibility, and reliability.

Rotary pumps, or rotary fans, are commonly used in electronic cooling, while axial and centrifugal fans are two majorities. The difference between axial and centrifugal fans is the directions of the inflow and the outflow. The inflow and the outflow are in parallel for axial fans, whereas the inflow is perpendicular to the outflow for centrifugal fans, as shown in Fig. 3.

where the subscripts represent different fans.

Given in the fan laws, flow rate and pressure are two indicators to represent the performance of a rotary fan. The curve of flow rate versus pressure (PQ curve) of a fan represents how far away the fan can deliver the air. Typically, axial fans are suitable for high flow rate applications, while centrifugal fans are suitable for high-pressure applications. On the other hand, connecting multiple rotary fans in parallel results in a higher flow rate, while connecting multiple rotary fans in series results in higher pressure.

Axial pumps, commonly known as axial fans, are one of the most common airflow generators used in electronics because of the low cost, good performance, and high availability. A typical axial fan consists of several components such as the motor, the rotor (with a few blades), and the housing, as shown in Fig. 4. When the motor is driven and the rotor spins, the sweeping blades impart energy to the air to generate airflow.

The performance of an axial fan depends on many factors such as the blade configuration (shape and number) [9–11], the housing design [12,13], the outlet structure [14], etc. Figure 5 shows the schematics of the splitter blades and the outlet strut designs. On the other hand, the rotor has been shown to affect fan performance as well, e.g., the number and the operation of rotors [15–17]. Figure 6 shows an axial fan with the contra-rotating rotors (two rotors).

It is known that axial fans create relatively low pressure when compared with centrifugal fans; however, connecting multiple axial fans in series could result in higher pressure. However, connecting axial fans in series could also cause a higher noise. Wang [18] analyzed the noise for the two identical axial fans connected in series. It is found that the inlet flow distortion increases the noise level. With the help of the flow straightener either at the inlet or in-between the two fans, the noise level can be reduced.

Although placing multiple axial fans in parallel results in a high airflow rate, the cooling performance of such parallel settings might not be increasing as desired. Stafford and Fortune [19] demonstrated that the cooling performance of a three-fan-sink could be dependent on the flow characteristics. The results show that, if the heat sink has only two-side exits, the cooling performance could be degraded because of the substantial cross-flow coming from the adjacent fans (Fig. 7).

When applying a fan in practice, especially in a small and compact system, various components could block the airflow pathway at either inlet or outlet of the fan, and as a result, the performance of the fan could be far different from the case with free airflow pathway. Lin et al. [20], Fukue et al. [21], Wang et al. [18], and Kang et al. [22] demonstrated that when the inlet of an axial fan presents some obstacles (the inlet blockage effects), both the flow rate and the pressure decrease. Anton et al. [23], on the other hand, showed that the outlet blockage affects only the flow rate of an axial fan. Wang et al. [18] also mentioned that the noise level also rises over the inlet blockage. Moreover, Manaserh et al. [24] reported that when bening placed inside a rack server, the performance of an axial fan can be degraded dramatically by the obstructions around.

Regarding reliability, Jin et al. [25] showed that the mean time to failure (MTTF) of a regular cooling axial fan could range from 40,000 h to 280,000 h at 60  °C. An in-house report shows that the estimated life expectancy (bearing life on the reliability of 90%) and the MTTF of one of its products (Intel Server Board S1200V3RP) could be higher than 88,000 h and 200,000 h at 40  °C, respectively.

Centrifugal fans, also known as blowers or radial fans, are commonly used in thin or space-limited electronics (e.g., laptops and set-top boxes) that require overcoming high pressure or bending the airflow pathway. Like axial fans, the performance of centrifugal fans depends on the blade design (Fig. 8) [26,27], the housing structure (Fig. 8) [28–30], the blade-housing interaction [28,31], etc.

The battery pack usually consists of several cells connected either in series or in parallel and is thus an important and major heat source in portable electronics. Therefore, cooling a battery pack is challenging because the high flow rate and the high pressure are both demanding. Kwon and Park [32] demonstrated a guide vane (Fig. 9) that converts a centrifugal flow to an axial flow such that both high flow rate and pressure could be satisfied simultaneously.

Conducting heat to the housing of a fan is also effective because the airflow could take the heat away in the closest fashion. Staats and Brisson [33] realized such an idea and proposed a centrifugal fan that has a heated housing (heat sink), as shown in Fig. 10. By doing so, the airflow directly sweeps across the heated surface. Different blade shapes were conducted to examine the effects on the fan performance, while the conclusion given is that the fan performance has only a weak dependence on the blade geometry.

To save space, Kim and Kim [34] integrated a multistage nonconventional centrifugal fan into a heat sink, as shown in Fig. 11. The measurements reveal that the flow characteristics of the centrifugal heat sink are identical to typical centrifugal fans, and the average velocities at the inlet and the outlet are linearly proportional to the rotational speed of the blades (rotor), i.e., it follows the fan laws.

Other than the characteristics of the fan itself, the conditions where the fan is applied also have certain effects on the cooling performance. Walsh et al. [35] showed that, to a cell phone application (Fig. 12), the blockages at the inlet and outlet of a centrifugal fan could significantly degrade the cooling performance. Stafford et al. [36] presented that the cooling performance of a centrifugal fan also depends on the flow conditions at the inlet and outlet, and concluded that the cross-flow at the inlet is more important than the benefits of the increased flow rates at the outlet.

The structure of a centrifugal fan is similar to that of an axial fan, and thus, the reliability of a centrifugal fan is also similar to that of an axial fan. An in-house report of a fan manufacturer shows that the life expectancy of a 50 mm square centrifugal fan is 50,000 h at 40 °C (Delta BFB0512VHD-F00, Taiwan). Another fan manufacturer reports a 60,000 h life expectancy at 40 °C for a similar 50 mm square centrifugal fan (Sunon MF50201V1-1B000-A99, Taiwan).

Paddle fans or hand fans are probably the most ancient beam-type fans. The actuator of an ancient paddle fan is usually the human hand. When the beam oscillates faster, the airflow is stronger, but the hand feels exhausting faster. Modern beam fans, as shown in Fig. 13, work in the same way as paddle fans, except the actuator is electric-powered. One of the most well-known and well-developed actuators is piezoelectric (PE). PE effect is the induction between mechanical stress (strain) and electrical voltage (charge) [37], i.e., applying an electrical excitation to a PE actuator induces mechanical deformation that bends the beam and thus generates airflow. The generation of airflow occurs in all directions around the beam, making the orientation and the placement of such beam fans flexible. On top of such, the form factor and the power consumption of a PE beam fan are usually small and low [38,39], respectively. Taking all these features, PE beam fans are often used along with heat sinks in cooling electronics, as shown in Fig. 14.

Given an excitation voltage, the performance of a PE beam fan depends on the frequency and the amplitude of the oscillating beam [39–41]. When the beam has high flexural rigidity, the oscillation frequency is high, but the oscillation amplitude is low. On the contrary, when the beam is long, the oscillation frequency is low, but the oscillation amplitude is high. Therefore, to have the best cooling performance, there is a tradeoff among geometry, dimension, the material of the beam, as well as the configurations between the fan and the heat source.

Lin et al. [42] and Li et al. [43] performed similar experiments and simulations to show how the beam shape affects the convective heat transfer for an open heated plate, as shown in Fig. 15. The results indicate that the first-order resonance frequency, the excitation voltage required to achieve the same amplitude, the vortex structure induced around the blade tip, and the local heat transfer around the blade tip differ from one blade shape to another. The results suggest that a wider rectangular blade shape (W₂/W₁ is 1) performs better in the cooling performance than a narrower one does, while the divergent shape (W₂/W₁ is 2) is not favorable in the convective heat transfer.

Sufian et al. [44] presented the effects of both the side and the tip gaps of a PE beam fan embedded in a three-side confined rectangular channel on the cooling performance, while the heat source is at the tip side, as shown in Fig. 16. The results show that the tip gap has stronger effects on cooling performance than the side gap has. The smaller gaps (both side and tip) result in higher cooling performance.

Other than being embedded in a heat sink, a PE beam fan can be placed outside of a heat sink either. Li and Wu [45] demonstrated the cooling performance of a pin-fin heat sink by an impinging jet generated by a dual PE beam fan, as shown in Fig. 17. The results suggest that operating these two beams in a counterphase makes the thermal resistance of the cooling systems lower. Gugulothu and Chamkha [46] demonstrated how using a dual PE beam fan affects the cooling performance of a plate-fin heat sink, but by a parallel jet, as shown in Fig. 18. The results also show that operating such a dual-beam PE fan in counterphase results in better cooling performance than in in-phase.

As shown in Fig. 19, Chen et al. [47] presented how the PE beam fan induced turbulence in a channel flow improves the cooling performance by looking at the ratio of the Nusselt numbers that are with and without exciting the PE beam fan. The results show that when the freestream has a higher Reynold's number, the Nusselt number ratio is up to 46% improved because of the stronger turbulence (higher flapping amplitude).

Furthermore, Tiwari and Yeom [48] simulated the flow field of a PE beam fan, as shown in Fig. 20, and concluded that the direct impingement or sweeping-induced vortex over the heat source is responsible for the cooling capacity in the channel. The reported maximum heat transfer enhancement resulting from the PE beam fan is 102% at the Reynolds number of 603.

Conway et al. [49] examined the effects of the beam thickness (1.0 mm and 3.7 mm) on the aerodynamic characteristics using both particle-image velocity and simulation. The results (Fig. 21) show that the beam thickness has a significant influence on the wakes within the oscillation plan. There are two different diverging jets induced for the 1.0 mm beam, while there are two strong lateral wakes generated at small flow disturbance downstream of the beam tip for the 3.7 mm beam.

Noise generation of a PE beam fan depends on many factors such as material, dimension, geometry, voltage, frequency, etc. A few literature report that the noise generated by a PE beam fan with sizes suitable for small heat sink applications could be around 35 dBA [50,51]. However, Petroski et al. [52] stated that most typical PE beam fans operate at the noise level of 25 dBA or lower.

Although the failure modes of the piezoelectric materials have been well discussed [53,54], there are limited academic discussions on the MTTF or the mean time between failures of the compact PE beam fans [55]. There are a few commercial PE beam fans advertised with the mean time between failures greater than 150,000 h (Piezoflo PFN-1011/1012) or the MTTF higher than 4600 h [56].

Figure 22 shows the schematic of a typical diaphragm (membrane) pump. The diaphragm that moves reciprocally up and down is used to change the pressure of the pump chamber, while the PE actuator is used to manage the movement of the diaphragm. When the diaphragm is ready to move up, the inlet valve is open and the outlet valve is closed such that the air is drawn into the pump chamber during the diaphragm's moving up. When the diaphragm is ready to move down, the inlet valve is closed, and the outlet valve is open such that the air is pushed out of the pump chamber during the diaphragm's moving down. Unlike beam fans, generating airflow in open space, diaphragm pumps require at least one chamber to create the pressure, and thus, the airflow. Despite diaphragm pumps are flexible in the chamber design, the performance of a diaphragm pump depends on factors such as the operating conditions of the PE actuator [57,58], the configuration of the chamber [57–59], and the structure of the inlet and outlet [60,61].

de Bock et al. [58] measured the PQ curves of a 40 mm by 40 mm by 1 mm dual-jet diaphragm pump using a standard flow bench, as shown in Fig. 23. The reported highest pressure and flow rate are ∼24 Pa and ∼28 liter-per-mintue when the operating voltage and frequency were 50 V and 182 Hz, respectively, while the PQ curve of a conventional blower was also presented for the comparison purpose. The results indicate that the performance of a well-designed diaphragm could outperform a conventional blower. The results also suggest that such a dual-jet diaphragm pump is a zero-net-mass-flow but positive momentum device such that restricting the inlet buffer does not affect the performance too much.

Liu et al. [62] showed how the flow rate of a diaphragm pump relates to the geometric factors, including the height of the inlet channel (h_i), the diameter of the outlet/jet orifice (d_o and d_j), and the height of pump chamber (h_p). The results suggest that the factors that affect the flow rate the most to the least are h_p, d_j, h_i, and d_o. The reported a maximum flow rate of 2.79 liter-per-mintue at the operating conditions of 150 V and 3.25 kHz (Fig. 24).

By using de-Bock's structure, Jalilvand et al. [63] demonstrated the cooling performance of such the diaphragm pump along with a heat sink for the central processing unit application, as shown in Fig. 25. The results suggest that such an ultrathin (1 mm) diaphragm pump could still provide enough cooling capability with the help of the heat sink.

Deng et al. [60] visualized the flow patterns around the outlet of a diaphragm pump and examined the Nusselt number when such a pump impinges on a heating foil at different distances. The results indicate that the flow spreads outward at 22.5 deg and 26.6 deg when the pump comes with the single- and dual-slot outlets, respectively. The Nusselt number would reach the maximum when the ratio of the distance between the pump outlet and the heating foil to the hydraulic diameter of the outlet is 5.5 regardless of the number of the outlets (Fig. 26).

Jeyalingam [61] indicated that the typical diaphragm pumps used in electronics could have a noise level of up to 73 dB. Nevertheless, by revising the orifice (inlet/outlet) shape of a two-chamber diaphragm pump from the conventional circular ones to the lobed ones that have the same effective diameter and area, the noise level could be reduced by 14 dB (Fig. 27).

Electrohydrodynamics (EHDs) is a physics involving electrostatics, fluid mechanics, and charge transport. Figure 28 shows the working principle behind an EHD pump. The corona electrode is of a relatively large curvature (e.g., needles or thin wires) and is applied a high voltage, while the collector electrode is of a relatively small curvature (e.g., plates or thick rods) and is electrically grounded. When the electric field around the corona electrode is large enough to induce corona discharge, incoming air molecules will be charged. When the charged molecules move to the collector electrode following the electric field, imparting charge and momentum to neutral air molecules, the resultant effect is the bulk air movement, also known as ionic wind or EHD flow.

The greatest feature of an EHD pump is no moving parts such that EHD pumps can be small form factor, low profile, simple configuration, silent, vibration-free, and low power consumption. The performance of an EHD pump is governed by the factors that affect the characteristics of the electric field around the corona electrode, including the physical and electrical factors like geometry/configuration [65–68], electrode material [69,70], voltage [67,71], current [64,72], and polarity [66,73].

Peng et al. [65] simulated a wire-plate EHD flow generation in a rectangular channel and presented how the physical factors affect the flow generation, and thus the heat transfer, as shown in Fig. 29. The results suggest that there is a threshold number of electrodes for maximizing the heat transfer enhancement, further increasing the number of electrodes does not help the heat transfer enhancement but increases the power consumption. On the other hand, when the number of corona electrodes is constant, the heat transfer enhancement can be improved with an appropriate pitch between adjacent electrodes. Both the above-mentioned phenomena can be attributed to the barrier effect that states the charge density around the corona electrode could be lowered for a multiple corona electrode system. Jang and Chen [71] made a series of simulations similar to what Peng et al. [65] did except the corona electrodes are needles, and the plate surface (electrically grounded) is at a constant temperature, as shown in Fig. 30. The results also conclude that there is an optimum pitch of corona electrodes for heat transfer enhancement and power consumption.

Moronis et al. [74] presented the characteristics of a two-stage wire-to-wire EHD pump (circular tube), including the noise level, the airflow rate, and the efficiency (airflow power over electrical power), as shown in Fig. 31. Compared with the traditional rotary fans, such a two-stage wire-to-wire EHD pump has a comparable airflow rate and efficiency but has significantly low noise and power consumption.

Wang and Wen [64] presented the PQ characteristics of the wire-to-rod EHD pumps with respect to the inlet blockage, as shown in Fig. 32. The results show that both the pressure and the flow rate decrease when there presents an inlet blockage. The inlet blockage affects the flow rate more than the pressure, opposite to the rotary fans [20]. Besides, the PQ curves better withstand the inlet blockage when the EHD pump is driven by constant corona current.

Ramadhan et al. [75] integrated a wire-to-plate EHD pump into a 2-mm-fin-pitch aluminum heat sink, as shown in Fig. 33, and demonstrated that the EHD pump is a promising thermal solution for the laptops. When the overall height and width of the heat sink are 12 mm (H_h) and 88 mm (W_h, 25 fins), such an EHD pump can keep the base temperature below 85 °C under a 30 W thermal design power.

Needles are the most common corona electrode used in an EHD pump. Sun and Velásquez-García [76] three-dimensional-printed the needle corona electrodes using stainless steel for a needle-ring EHD pump. The results show that the average tip diameter for an as-printed needle and an electropolished needle are 300 μm and 83.4 μm, respectively. The generated EHD flow velocity can be improved by 22% when the needle is electropolished (Fig. 34).

Tsui et al. [77] showed a needle-to-plate EHD pump used to impinge on a 7.5 W heat source (plate, 70 mm × 50 mm × 3 mm) and got the heat transfer coefficients 2.6–4.8 times higher than the natural convection. Qu et al. [78] demonstrated a needle-to-ring EHD pump used to cool a 10 W ligtht-emititng diode (LED) that comes with a heat sink and showed the maximum temperature difference at the heating film center between the cases with and without the EHD pump could be 35.6  °C. Feng et al. [79] presented a needle-to-plate EHD pump, where the plate is the fin of the heat sink, and reported that the maximum temperature drops from 54.5  °C to 39.1  °C for a 7.2 W heat source while the EHD pump consumes 0.85 W power only. He et al. [80] proposed a wire-to-plate EHD pump for a thermoelectric refrigeration system and reported that the EHD cooling system makes the coefficient of performance of the refrigeration system similar to that using traditional mechanical fans while the noise level is only 4.2 dbA (Fig. 35).

Air cooling, especially forced convection, is the most popular and economical way to manage the thermal issues of electronics. The primary purpose of an air pump is to initiate and maintain airflow, i.e., to sustain the convection. While every type of air pump has unique strengths and applications, Fig. 36 is a radar chart that scores the reviewed five types of air pumps with respect to flow rate, pressure, noise, flexibility, and reliability (Table 1).

There is no doubt that rotary pumps (axial fans and blowers) are the most widely used air pumps for electronics because the flow rate, pressure, and reliability are sufficient. However, although rotary pumps are mature and well-developed, rotary pumps may not be appropriate when coming to small electronics or particular applications (e.g., silent cooling).

Beam fans usually operate in an open space with moderate airflow velocity but low pressure, while diaphragm pumps are able to create huge pressure but low airflow velocity. Nonetheless, beam fans and diaphragm pumps can be thin or highly customized to fit into small spaces such as mobile devices and LEDs [86–88]. Yet, beam fans and diaphragm pumps consist of moving parts and operate at a high frequency and therefore the noise is inevitable, especially diaphragm pumps because of the generation of high pressure [61].

On the other hand, EHD pumps are flexible in the electrode design so that EHD pumps can be configured in a small or irregular space. Besides, EHD flow is also known to be useful in the boundary layer control [89,90], and thus the heat transfer enhancement [91,92]. Furthermore, EHD pumps have no moving parts, making the silent operation possible [80,93]. However, EHD pumps suffer from performance degradations because of the oxidation on corona electrodes [94,95], even coating nanomaterials (e.g., monolayer graphene and single-walled carbon nanotubes) onto corona electrodes could ease such the problems [96]. Additionally, ozone is a by-product of the EHD pump (because of corona discharge) and is not favorable to people's health [97–99].

Managing thermal issues are becoming more and more important in electronics because the form factor is getting smaller while the power consumption is going higher. Air forced convection is a simple, efficient, and cost-friendly solution, while air pump is a component to initiate and sustain the forced convection. Other than two state-of-the-art air pumps (axial fans and blowers), this paper also reviews three emerging air pumps (beam fans, diaphragm pumps, and EHD pumps) that have a great potential in electronics, especially those having space and noise concerns.

When evaluating an air pump used in a thermal solution, the flow rate and pressure are certainly essential. However, many modern electronics may ask for more, such as low noise and high reliability. Form factor, placement, and orientation could be critical as well, especially for small or portable electronics. Additionally, one should also take the cost, availability, maintenance, and even appearance (e.g., gaming electronics) into account. Therefore, it is conceivable that developing emerging air pumps would become more and more challenging but would be beneficial in pushing the electronic industry a great step forward.

The authors would like to thank Mr. Po-Hao Wang for the article collection and Ms. En-Ying Jiang for the drawings.

• Ministry of Science and Technology, Taiwan (MOST 108-2221-E-011-055; Funder ID: 10.13039/501100004663).