Study On Heat And Mass Transfer Of Thermal Protective Clothing And Prediction Of Skin Burn Upon Hot Liquid Splashes

The emphasis of study on thermal protective materials and clothing is one of the most important approaches to improve national safety development and promote the textile industry. The current studies on thermal protective clothing mainly focus on flame, radiant heat and contact burn. However, the workers in firefighting, petrochemical, and food processing industry will also encounter the hazards of hot liquid splashes and steam. Weather the traditional thermal protective clothing can provide sufficient protection against hot liquid splashes to ensure the occupational health and safety of workers is a hot topic of public health. The protection provided by protective clothing not only depends on fiber and fabric thermo-physical properties, but also relates to garment structure, design features, garment fit and etc. To achieve the above requirements and objectives, the heat and mass transfer through protective clothing under exposure to hot liquid splashes was investigated, and the skin burn injury was predicted. Firstly, a novel bench scale hot liquid splashes tester was developed to investigate the effects of fabric properties, exposure liquid type and air gap on the thermal protection provided by protective materials used for protective clothing; furthermore, the effects on the impact liquid penetration through protective fabrics were explored; and thus the heat and mass transfer through protective materials was clarified. Secondly, a novel method was proposed to characterize the air layer entrapped in thermal protective clothing, and the air gap size and distribution was analyzed; an advanced instrumented spray manikin test system was established to investigate the thermal protection of full scale protective clothing, the effects of material properties, design features and garment size on heat transfer and skin burn injury were investigated. Finally, the correlation between bench scale test and manikin test was analyzed, and the prediction model of overall protective performance was established. The specific research contents were provided as follows.1)Thermal performance of protective fabrics upon hot liquid splashesIn this study, a new hot liquid tester was developed based on the modification of standard test device described in ASTM F2701.The most important innovations include temperature-controlled circulating reservoir, liquid delivery pipeline, flow control component, skin simulant sensors and skin burn prediction program.Seven typical used protective materials in North American market were selected in this study. The protective performance was evaluated by the newly developed apparatus, and the mechanism associated with heat and mass transfer was clarified. Under exposure to hot liquid splashes, the heat transfer modes consist of convection on the fabric surface, thermal conduction and radiation in the fabric and energy transfer associated with mass transfer. The results indicated that impermeable and semipermeable fabrics provided better performance than permeable fabrics, and the fabric thickness greatly affected the protective performance of impermeable and semipermeable fabrics; for the permeable fabrics, the decrease of liquid penetration, could significantly improve the performance of permeable fabrics. It was demonstrated that the mass transfer was one of the key factors influencing the heat transfer and skin burn injury. The surface property of fabric and dynamic viscosity of liquid affected the liquid penetration. Different liquids caused distinct damage to skin, namely the drilling fluid and distilled water resulted in more severe skin burn injury under the same exposure configuration. Thermal diffusivity of liquid, mass transfer rate and total amount of penetration determined the overall protective performance of a fabric system. The skin burn injury at the position under the liquid jet was more severe than other areas, which might be related to impingement force due to liquid jet and liquid penetration performance.2) Factors affecting liquid impact penetrationBased on the standard AATCC42-2000"Water Resistance:Impact Penetration Test" and the newly developed hot liquid splashes tester, a new approach to evaluate the liquid penetration under exposure to hot liquid splashes was proposed.The effects of fabric properties, liquid temperature, liquid type, impingement angle, and fabric combinations on liquid impact penetration were investigated. The results indicated that the fabric with higher wetting resistance provided lower liquid absorption and penetration. The liquid penetration increased with the liquid temperature. The dynamic viscosity of liquid greatly affected the penetration, furthermore, the challenge liquid with bigger viscosity caused lower penetrated and higher stored amount of liquid. The impingement angle also showed significant effect on the liquid transfer performance, namely the smaller inclined angle resulted in larger penetrated and absorbed amount of liquid. The addition of thermal liner sharply increased the stored amount of liquid, decreased the penetration; moreover, the involvement of moisture barrier caused more liquid stored in the outer shell, but the total stored amount of liquid was significantly lower than that with thermal liner.3) The effect of air gap on heat and mass transfer of protective fabricsThe air gap has important effect on heat and mass transfer of protective clothing. In this study, an air spacer of6mm was made and added between the fabric and the sensor board to investigate the effect of air gap on thermal protective performance against hot liquid splashes. The results showed that the energy transferred to skin depended on the fabric properties, liquid properties and test configurations. Different liquids caused different damage to skin, which was related to the test condition. The air gap sharply decreased the absorbed energy, extended the second degree burn time, and thus improved thermal protective performance. It was confirmed that keeping the air layer between garment and human skin was a very effective approach to provide high performance. The effect of air gap on protective performance depended on the fabric type. The air gap could decrease the liquid penetration and vapor transfer through permeable fabrics, and thus significantly improve the protective performance. However, for the hydrophobic permeable fabrics, vapor could penetrate through the fabric and condense on the sensor with discharging of energy to sensor, and thus sharply eliminated the positive effect of air gap.4) Characterization of air gap in thermal protective clothingTen typical thermal protective clothing for industrial workers was selected, and both the nude and dressed manikin was scanned by Human Solution3D body scanner. The data was processed by a novel developed procedure using Rapidform XOR and XOV software. The local and overall average air gap size and its distribution were analyzed in terms of material properties and garment size.It was indicated that the air gap between garment and human skin depended on the body geometry, fabric properties and garment size. The air gap size in the convex area was higher than that in concave area, and the regions with larger circumstance presented smaller air gap. The garment with bigger fabric weight showed larger air gap size. The flexural rigidity also affected the air gap size, namely the garment with higher rigidity fabric presented worse drapability, and thus the air gap was bigger. The larger garment size showed higher local and overall average air gap size. In addition, the air gap distributed unevenly over the human body; the air gap at leg and abdomen was relatively high, while the air gap at chest, hip and arms was relatively low.5) Protective performance evaluation using instrumented spray manikinA new instrumented spray manikin test system was established based on the flame manikin test system, which was the only apparatus to evaluate the protective performance of protective clothing upon hot liquid splashes. The effects of fabric properties, design features, and garment size on heat and mass transfer and skin burn injury distribution were investigated. The newly developed spray manikin test system was proved to be capable of differentiating the selected protective clothing in terms of absorbed energy and percentage of skin burn. The tested protective clothing was divided into seven categories according to the protective performance. The impermeable and semipermeable garments provided better performance than those of permeable garments. The thickness of fabric and design features showed effects on the performance of impermeable and semipermeable clothing. The effect of fabric weight on heat transfer through protective clothing system seems to be minimal.The water repellency treatment showed great effect on protection.The size of garment showed effects on thermal protection of permeable garments, but these were not significant. The difference between the tight clothing and the loose clothing was significant, but there was no significant difference between the fit garment and the tight or loose garment. Minimizing mass transfer was recognized as the critical factor for protection from hot water. Maintaining a proper air gap between the garment and human body was a critical factor in improving thermal performance. The design features also affected the overall performance. Adding fabric layers could improve thermal protective performance of garments. The reflective tape could provide extra protection and decrease skin burn injury.The skin burn injury distributed unevenly, and mainly occurred at the areas of compression upon water spray (chest and abdomen), heavy water flow (waist, hip and leg) and small air gap.The overall average air gap size showed significant negative correlation with the percentage of skin burn and total absorbed energy. Generally, the air gap distribution and skin burn distribution presented negative relationship. The negative correlation between local average air gap size and skin burn injury was presented. Moreover, the relationship was significant except pelvis (P<0.05). For a specific garment, the air gap at a certain area didn’t correlate well with the average air gap size. The air gap size at upper extremity negatively correlated with the skin burn injury, except the garment G5.6) Prediction of performance of protective clothing upon hot liquid splashesThe correlation between the parameters obtained in bench scale test and manikin test was conducted. The results showed that the second degree burn time significantly linearly correlated with the absorbed energy in bench scale test without air gap, and the percentage of skin burn was also significantly linearly related to the absorbed energy; the indices obtained in bench scale test without air gap were significantly nonlinearly correlated with those measured in manikin test respectively; the correlation was linear when there was a6mm air spacer. In addition, the prediction model of percentage of skin burn was established based on the second degree burn time and average air gap size, namely PBI=97.671-2.597*T2-1.84*AAG. This model indicated that the percentage of skin burn could be predicted if the second degree burn time of the fabric and the average air gap size of the garment were known.