基于混合量热原理的冻土比热容测试方法

土的比热容是冻结法施工中的重要参数,但既有混合量热法得到比热容是某一负温到平衡正温这一阶段的平均比热容而不是该负温点的比热容。根据黏土在冻结过程中孔隙水的相变随负温增加逐渐发生的客观事实,基于传统混合量热法建立了冻土比热容的递推算法。首先,待测试冻土试样分别由某一负温的左右两个微小增量开始,经混合量热法各个步骤后到达热平衡状态。则由负温开始至热平衡状态,试样吸收的热量Q,必然等于负温至0℃和0℃至平衡温度这两个阶段热量交换Q_1和Q_2的代数和。由于不存在相变,试样从0℃至热交换平衡温度需要的热量可以由常规的混合量热法获得。因此,试样由负温开始至0℃的热量可以通过两者相减得到,并可进一步得到试样由该负温左侧增量升温至该负温右侧增量需要的热量。最终,试样在该负温点的比热容可以根据比热容的定义得到。本文建立的冻土比热容递推算法能得到某温度点的比热容而非某温度段的平均比热容,且包含了潜热的贡献,因而更为合理有效。

Test methods for specific heat capacity of frozen soil based on principles of mixing calorimetry

The specific heat capacity of soil is an important parameter in the ground freezing method. However, the specific heat capacity obtained by the exiting mixed calorimetric methods is the average specific heat capacity from a negative temperature to the equilibrium positive temperature rather than that at the negative temperature. According to the fact that the phase transition of pore water occurs gradually with temperature change in freezing process, a recursive formula to the specific heat capacity of obtain frozen soil is established based on the exiting mixing calorimetry methods. First of all, two small equal temperature increments, a negative and a positive, are set for the negative temperature of the frozen soil. Then, two samples are prepared respectively at the two temperatures and the mixing calorimetry method is performed in order to arrive at each thermal equilibrium state. From the negative temperature state to the thermal equilibrium one, the heat absorbed by the sample, Q, will be equal to the total aequum of the two stages, Q₁ for negative temperature to 0℃ and Q₂ for 0℃ to equilibrium temperature. Since there is no phase change and latent heat, the heat required by the sample from 0℃ to the equilibrium temperature can be obtained by the conventional mixing calorimetry. Therefore, the required heat for the sample from negative temperature to 0℃ can be obtained by subtracting Q₂ from Q. And Further more, the required heat can be obtained for the given temperature from the negative increment to the positive increment. Finally, the specific heat capacity of the sample at the negative temperature can be obtained according to the definition of the specific heat capacity. From the proposed method, the capacity at any temperature, other than an average over a temperature range, can be calculated. And the proposed method can take latent heat into account. Therefore, it is more reasonable and effective.