# Design of Liquid-Air Hybrid Cooling Garment and Its Effect on Local Thermal Comfort
## 1 Introduction  
Indoor personnel usually use cooling systems such as air conditioners and electric fans to cool down in hot environments. However, such cooling sources are equipment-specific, importable, inflexible, and poorly adjustable to the needs of different individuals. Also, the use of air conditioners raises ecological and environmental issues [1]. Since these cooling sources limit the scope of personal use, the study of cooling clothing has been of increasing interest to researchers in the past decade. These garments can make up for the lack of thermal regulation of the human body in a hot environment and provide additional thermal protection to the wearer. In 1962, [PERSON] and [PERSON] [2] developed the first human-cooling garment, which became the starting point for the exploration of individual micro-environmental cooling clothing systems. Cooling clothing includes the use of air, liquids, phase change materials, and other cooling media to regulate the temperature of the micro-environment under the clothing to reduce the heat stress of the body in high-temperature environments [3]. The cooling form of these clothing items mainly includes liquid cooling, with a liquid pipeline to cool the body; ventilation cooling, with micro-fans to cool the body; PCM (phase change materials) cooling, with PCMs to cool the body; and hybrid cooling with both PCMs and ventilation. Compared with other cooling technologies, such as PCM cooling and air cooling, liquid cooling has higher reliability and cooling efficiency [4]. Currently, liquid cooling garments have been studied by many researchers in the aim of reducing thermal stress. The concept of something being \"wearable\" is also proposedand applied in the field of possible solutions developed in order to monitor the thermal state of the body [5]. [PERSON] [6] designed a liquid cooling garment where the pipe was woven into the garment to carry cold water to promote heat dissipation. [PERSON] [7] et al. explored the cooling effect of 10% and 20% concentrations of phase change micro-particle coolant and water at different combinations, studied the effect of the inlet temperature and the flow rate, and concluded that the cooling effect of the phase change micro-particle solution was superior to that of water, especially at an inlet water temperature of 13 \({}^{\circ}\)C, when it can achieve the best effective cooling. [PERSON] et al. [8] designed a man-portable cooling garment with cold liquid circulation based on thermoelectric refrigeration, and the results indicated that the system was capable of providing a water temperature of 15.7 \({}^{\circ}\)C and a cooling power of 340.4 W, with a climatic chamber temperature of 30 \({}^{\circ}\)C. In addition to the use of liquid-air garments in daily life, there are also applications in the medical and health fields, the military, and the firefighting field. [PERSON] et al. [9] used three different cooling garments to compare the response of patients with multiple sclerosis (MS) to short-term cooling therapy. The results showed that the active cooling garment system was the most effective one for reducing the temperature of MS patients. [PERSON] et al. [10] tested the effectiveness of liquid cooling undergradments (LCUs) in reducing the heat burden of soldiers while wearing chemical protective clothing. They found that the cooling effect of LCUs could be increased by designing a larger body coverage for cooling. [PERSON] et al. studied a liquid-air garment for managing heat stress in firefighters and found that the liquid-cooled garment could achieve 70% cooling within the first ten minutes [11]. [PERSON] [12] et al. designed two liquid-air garments with different compositions and placed them on a thermal manikin for cooling comparison, but both were limited in their area of use due to the heavy cooler of the liquid-air garment. At the same time, portable ventilated garments have the advantages of convenience, light weight, and low cost [13].