Improving air cooling efficiency of transmit/receive modules through using heat pipes
Introduction
Modern radar stations are widely used to obtain high spatial resolution images of the surface of the Earth. This helps to solve problems of weather forecasting, search for natural resources, geological exploration, bioresources assessment, creating topographic maps, monitoring of disasters and environmental pollution, etc. Radar stations also make it possible to detect moving objects on land, on water and in the air, and to determine coordinates and parameters of their movements with high accuracy.
In recent years, active phased array antennas (APAA) have been widely used as antenna systems in order to expand the functionality of radar stations [1]. The APAA technologies are constantly perfected due to advances in solid-state microwave integrated circuit development and design [2]. An APAA includes a large number (from tens to several thousand) of transmit/receive (T/R) modules [3]. In some APAA versions, the T/R modules are made as separate units [4], [5].
Receiving modules and receiving channels of T/R modules usually do not emit large amounts of heat, unlike transmitting modules and transmitting channels of T/R modules, which generate quite a lot of heat. Most of the heat is released by the output amplifiers of the transmitting channels. By design, an output power amplifier consists of a base made of heat-conducting material (usually aluminum alloy), cut into one side of which are recesses with shielding walls. These recesses are used to mount microwave elements [6], [7]. The electronic elements mounted into the recesses are hermetically sealed with covers.
Other power amplifier designs have longitudinal cooling fins on the lower side of the heat sink base for better heat dissipation. The amplifier described in [7] has a heat sink base with 17 fins (24 mm heigh and 2 mm thick) along half the length of the case. The heat sink is ventilated with air flow at a velocity of 0.5 m/s, which ensures that the temperature of the amplifier case does not rise above 85 °C at an ambient temperature of 20 °C. The output power of such an amplifier in continuous mode is 11 W [7].
In order to increase the heat dissipating ability, Somsing Rathod et al. [2] used a heat sink with petal-shaped fins in the design of their T/R module, and Baranuk et al. [8] tested a heat sink with plate fins cut into petals rotated at an angle of 30°. Installed on the heat-generating elements of the T/R module and ventilated by a flow of cooling air, such heat sinks help reducing the temperature of the active microwave elements.
In other T/R module designs [9], [10], the problem of local heat flux distribution over a larger surface of the heat sink is solved by mounting several active heat-emitting microwave elements (transistors or monolithic integrated circuits) in thermal contact with a common copper heat flux diffuser. Such copper heat flux diffusers with microwave elements and printed circuit boards installed on them are known as amplifying submodules or pallets [11], [12]. One T/R module may include several pallets. The pallets are then mounted on a cooled base of the module made of aluminum alloy.
The heat flux from the microwave elements can also be diffused with metal [13], ceramic [14] or silicon [15] vapor chambers.
The transition from gallium arsenide to higher-frequency gallium nitride electronic components when designing new and modernizing the already developed T/R modules for APAAs made the problem of increasing the cooling efficiency of active microwave elements of output power amplifiers particularly relevant. Gallium nitride elements are characterized by higher specific and total heat release values. While the power level of gallium arsenide based T/R modules is about 10 W [6], the values for the modules on gallium nitride reach 15–20 W and more [16], [17]. The heat generated by the active microwave elements of the output power amplifiers of T/R modules leads to an increase in the temperature of the active elements, a decrease in their reliability, and a decrease in the output power of the signals [18]. Therefore, in the development of new and modernization of the existing T/R module designs, special attention should be paid to the issues of improving the cooling efficiency of active microwave elements.
A promising technical solution for reducing of the temperature of the mounting surface of the supporting base of the air-cooled T/R module case can be to embed heat pipes into the base of the heat sink case so that the heat-generating elements of the output power amplifiers are mounted in thermal contact with the evaporation zone of the heat pipe, while the condensation zones of the pipes are in the finned area of the supporting base [19]. There is a wide variety of heat pipes with different types of wick: sintered powders [20], segments of metal fiber [21], [22], powder and fiber combined within one wick structure [23], screen mesh wicks [24], [25], longitudinal [26], [27] or threaded [28], [29] grooves, etc. The most suitable type for use in T/R modules are the heat pipes with a rectangular or flat-oval transverse cutting of the envelope [20].
The effective thermal conductivity of heat pipes is orders of magnitude higher than that of such metals as copper and aluminum [30]. Thus, embedding heat pipes into the base of the case allows distributing the local heat flux from microwave transistors over the entire finned heat sink surface with a minimum temperature difference along the length of the fins, regardless of their distance from the heat source, thereby increasing the heat dissipating capacity of the fins and further reducing the temperature in the mounting spots of microwave transistors.
The efficiency of heat pipes under the condition of mechanical effects [31], [32], [33] allows them to be used in T/R modules mobile radar stations.
The goal of this study was to evaluate the thermal mode of the basic design of the T/R module case with eight powerful active microwave elements installed directly on its mounting surface, and to explore possible ways to increase the cooling efficiency using heat pipes without resorting to manufacturing and experimental study of an expensive test sample of the product.
Section snippets
Research method
In order to achieve the set goal, the authors used numerical simulation, which is used rather widely in the studies of thermal and aerodynamic processes in heat removal devices [8], [34], [35], [36]. Numerical simulation reduces the designing cost of the thermal management devices for electronics, since it allows avoiding the manufacture of expensive experimental samples that need further experimental testing.
When performing numerical simulation for this study, the authors kept to the following
Choosing the cooling system and the basic T/R module design
A required thermal mode of the T/R module can be maintained using air [42], [43], [44] or liquid [45], [46] cooling systems. Liquid cooling systems are much more efficient and are used in cases when the air-type systems can not maintain the necessary temperature level of the active microwave elements. At the same time, using a heat, transfer liquid imposes a number of additional requirements on the APAA design, i.e., the design needs pumps for pumping heat transfer liquid and heat exchanging
Thermal model of the basic T/R module case design
Structurally, the T/R module in its basic version presents a rectangular heat sink case (Fig. 1). The geometric and power characteristics of the T/R module are shown in Table 1.
The mounting side of the supporting base of the module case is designed for the installation of electronic components and units. The most heat-generating electronic components of the T/R module are 8 microwave transistors of the output power amplifiers of the transmitting paths. Each microwave transistor emits 28 W of
Numerical simulation results for the basic design of the air-cooled T/R module case
Fig. 3 show the results of the temperature field numerical simulation for the surface of the base of the T/R module case containing eight mounting spots for heat-generating microwave transistors with a total emission power of 224 W. As can be seen from the figures, the highest temperature value at the mounting spots for the transistors is observed at an air flow velocity of 2 m/s in the interfin channels. At the same time, the maximum temperature at the mounting spots for microwave transistors
New design solution for T/R modules of APAAs using heat pipes
Fig. 4 shows a new design of the T/R module case with eight flat heat pipes embedded in its base.
The T/R module case is made of heat-conducting aluminum alloy. It contains a base 1 with two sides 2 and 3. Side 2 is the mounting side. It has 4 spots for the installation of heat-generating active microwave elements, electronic components and units (the elements themselves are not shown in Fig. 4). The reverse side 3 is the heat exchanging surface of the T/R module. It has longitudinal cooling
Temperature field simulation results for the T/R module case with heat pipes. Comparison of the obtained results with those for the basic design of the case without the heat pipes
In order to determine the temperature reduction efficiency for the mounting surface of the case base (Fig. 4), a numerical simulation of its temperature field was carried out with the same cooling air parameters (inlet temperature, air velocity in the interfin channels) and geometric characteristics of the case (overall case dimensions, number, height and thickness of cooling fins, distance between the fins) as the basic module design has. To numerically simulate the new design of the T/R
Conclusion
Numerical modeling of the basic version of the air-cooled heat sink case of the T/R module with a total heat output of 8 microwave transistors equal to 224 W and a new version of the heat sink body with heat pipes made it possible to evaluate the potential for reducing the maximum temperature at the mounting spots for the microwave transistors by means of both increasing the speed of air flow in the interfin channels and adding the heat pipes to the design. The results of the simulation allowed
Declaration of Competing Interest
The authors declare that they have no competing interests.
Acknowledgments
The authors would like to thank the Ministry of Education and Science of Ukraine for the financial support to this study (project No. 2114).
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2022, Case Studies in Thermal EngineeringCitation Excerpt :Moreover, topology optimization methods have also been used to design liquid cooling systems [27–29]. Other cooling methods, such as heat pipes [30–32] and phase-change materials [33,34] have also been used in array antennas. Different from the condition in the communication system, in the MWPT system, due to the high-power continuous wave transmission system, the thermal power consumption of the transmitting antenna is much larger, and the requirements for heat dissipation are more stringent.