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Analysis of The Effects of Thermal History of The Strata upon Temperature Variations in Underground Airways
AdvisorDanko, George L.
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An accurate underground mine Ventilation-Thermal-Humidity environmental simulation tool is essential for identifying and solving heat issues as well as preparing for long-time mine planning and design. There are various mine ventilation and climate simulation programs in the market. However, only a few of them have the capabilities to simulate dynamic changes of airflow, temperature, humidity, contaminants, and concentrations. Significant modeling errors may occur if dynamic transport in the strata wall components of energy (heat) and mass (contaminant and quantity) are not represented appropriately with time and distance. This research work reviewed the currently available commercial mine ventilation software models, usually with a step-change-type thermal solver, as well as an advanced model that has a “thermal history effect” (also known as thermal flywheel effect) solver method in MULTIFLUX (MF). The goal was to understand how large modeling error occurs if the model does not use the correct, spatially- and timewise- available temperature field in the calculation and, therefore, lacks the ability to deal with the “thermal history effect.” Two models were compared: one with and one without the thermal history component to process past and current temperature flow data.Five steps are taken in the study for testing and quantifying the problems above. The first step is to compare two models using constant intake air temperature. The goal is to show that any thermal simulation software with no thermal history component in the model will give erroneous results in long mine airways even if the input air temperature at the intake is taken as constant with time. The second step is to compare the no thermal history model using the maximum or minimum constant intake air temperature (currently the popular modeling approach) versus the thermal history model using the dynamic real input air temperature. The goal is to prove that a model without the thermal history element will give unpredictably erroneous results in long mine airways with time even if the constant maximum or minimum input temperature is taken from the real dynamic temperature range. However, it is hypothesized that a model with no thermal history element may provide a reasonable result for the average value of the temperature field with time when compared to the true, dynamic model output. This hypothesis is intuitively and widely used when, for example, the daily average temperature is used as input (replacing the hourly varying temperature data) for a yearly temperature analysis for seasonal changes. One may speculate that the number of the periodic cycles must be significantly large relative to the total time period of the thermal analysis.The third step of modeling test is to try to eliminate the model error by model calibration. However, the calibration result is depending on when and where the calibration is made; the “calibrated” model remains a model with the unpredictably uncertain outcome: either too “conservative” or “optimistic.” Note that “calibration” may work better for matching the long-term average temperatures. However, the proof of this hypothesis is left for the interested readers.The fourth step is a specific task for finding a simple and accurate input air temperature model. The goal is to express an analytical temperature model with only a few constants based on publicly available meteoric temperature data for the mine area. An analytical air temperature model is a helpful addition to a ventilation model for mine ventilation design. In addition, it is useful for analyzing and quantifying the model errors if a certain software lacks thermal history component.The fifth step is to use the analytical function as input air temperature in the comparison of the two models (one with, and one without the thermal history component). The goal of the study is to show that the thermal simulation software with no thermal history model component will still give erroneous results in long mine airways even if it is able to use the dynamic input air temperature. The test examples show that the dynamic input air temperature, and thermal history component are essential factors and could cause serious modeling accuracy problems if the model cannot handle them properly. An accurate Ventilation-Thermal-Humidity model with thermal history component is recommended. A method and an analytical function are given to simulate the dynamic air temperature in the mine airways for mine ventilation design and modeling tests.Keywords: Mine Ventilation, Thermal history, Air temperature, Modeling