【作者】 罗云；

【导师】 张腾飞；

【作者基本信息】 大连理工大学 ， 供热、供燃气、通风及空调工程， 2016， 硕士

【摘要】 多孔材料因其良好的隔热性能,被广泛用作保温材料。但多孔材料在高湿环境下易吸收水分,致使保温性能下降,且引发霉变。若能对多孔保温材料内积聚的水分含量进行实时监测,就方便采取及时的维护和保养措施,减小水分累积造成的危害。目前多孔材料内水分含量的测试方法很多,但大多集中于对土壤内部水分含量的测定,这些方法还难以兼具快速、廉价、无损、精准等特点。和土壤相比,多孔保温材料往往具有轻质和憎水的特点,其内部含水量常常远小于土壤含水量,且积聚的水分在其内部分布不均。对多孔保温材料内水分的精确测量具有更大的挑战。本文提出一种依据较短时间内的瞬态温变效应,来推测多孔材料内含水量的方法。测试时,在被测材料内布置电热线和测温传感器,给电热线供电产生恒定发热热流,并记录测温传感器的温升响应。根据考虑了发热体半径和热物性的经典复合导热理论,通过枚举方法获得与实验测得的瞬态温升响应最匹配的材料的容积热容值(即材料密度与比热容的乘积)。根据容积热容的叠加原理,多孔材料内的含水量与材料吸湿前后的容积热容值的改变量一一对应,由此推断出材料的含水量。本文选定轻质海绵为测试材料,测试材料内的体积含水量在0到80kg/m3。实验时将电热线和测温传感器的距离布置为10.5mm,对测得的100 s的温度响应进行匹配以求得材料的含水量。为评估本方法的测试精度,将本方法获得的含水量与称量法测得的含水量相比较。此外,还应用国际标准化组织(ISO)与国际计量局(BIPM)推荐的测量误差分析方法,评定了实验测试的不确定度,总结出可进一步提高实验测量精度的措施。两组独立测试的水分结果表明,本方法获得的材料含水量与称重法测得的材料含水量较为接近,两者之间的最大差异值为7 kg/m3左右,相对偏差集中在15%左右,基本可以满足工程测量的需求。由于称重法测量结果为材料整体的平均含水量,而本方法测得的含水量为发热体和测温传感器周围的局部含水量,当水分分布不均匀时,两种方法获得的结果会存在一定差异。通过不确定度分析可知,水分含量的不确定度受发热体热流、发热体与传感器之间的距离、温度测量三者精度的影响。当同时考虑以上三个因素的不确定度时,得到的材料含水量的不确定值的范围较大,称重法所得含水量落在不确定值的范围内。此外,通过降低热流和温升的相对不确定度能有效减小含水量的不确定度,同时固定发热体与测温传感器距离也有助于提高测试结果的可靠性。更多还原

【Abstract】 Porous materials are widely used for thermal insulation. However, porous materials can easily absorb moisture when exposed to high-humidity conditions. The gained moisture degrades the insulation performance and induces microbe growth. If the water content inside the porous insulation material can be routinely monitored, the maintenance and treatment to the insulation material can be appropriately taken to minimize the adverse impacts caused by the moisture accumulation. Numerous methods have been developed to detect moisture contents in soils. So far there is still lack of a rapid, inexpensive, nondestructive and accurate method for moisture measurement. As compared with the soils, the porous insulation materials are non-hygroscopic and light. The insulation materials cannot hold moisture with a quantity as large as that in the soils. The distribution of the moisture in a non-hygroscopic material can be highly non-uniform. Measuring the moisture in insulation materials is hence more challenging.This investigation proposed to measure moisture contents in porous materials based on transient temperature responses for a short while. An electric heating wire and two temperature sensors are embedded into the test material. The transient temperatures caused by the sudden constant heating of the heating wire were monitored. Based on the composite heat conduction theory, the impacts of the finite radius and heat capacity of the heating element to the monitored temperature responses are considered. The volumetric heat capacity (product of the density with the specific heat) of the material is obtained by matching the transient temperature responses through enumerating all possible thermal properties. According to the addition of volumetric heat capacities, the moisture content is inferred from the change of the volumetric heat capacity with respect to its dry status. In this investigation, light sponge blocks were chosen as the test material, whose volumetric moisture mass ranges from 0 to 80 kg/m3. The separating distance between the heating wire and the temperature sensors was fixed to 10.5 mm. The temperature responses within the initial 100s were recorded. To evaluate the precision of this method, the measured moisture contents between the proposed method and the gravimetric method using a digital precision balance were compared. In addition, the measurement uncertainties are evaluated by following the ISO and BIPM guidelines. Some strategies to improve the measurement accuracy are summarized.The results of two repeating series of isolated tests show that the maximum gap of the measured volumetric moisture mass between the proposed method and the gravimetric method is within 7 kg/m3 and most of their relative deviations fall within 15%, which is sufficient for engineering applications. Because the gravimetric method indicates only the averaged moisture contents in the whole material while the proposed method measures the moisture contents in the surrounding of the heating wire and the temperature sensors, the results may differ when the distribution of water is not uniform. The uncertainty analysis shows that the accuracies of the measured moisture contents are ascribed to the precision in measuring the heating power rate, the distance between the heating wire and the temperature sensor, and the transient temperatures. The possible moisture contents span widely after accounting for the above three uncertain sources. Fortunately, the moisture contents by the gravimetric method fall within the uncertain scopes. To further improve the measurement quality, a more precise powermeter and a more sophisticated temperature sensor are needed. In addition, maintaining a fixed distance between the heating element and the temperature sensor would help greatly to reduce the uncertainties更多还原