Study Of Heat And Mass Transfer Of Hydronic Snow Melting Process For Pavement

Hydronic snow melting is a new type of road maintenance technology developed in recent years. It plays an important role in traffic safety, energy saving and environmental protection. Aiming at hydronic road snow-melting systems based on renewable energy including solar and geothermal energy, the behavior of heat and mass transfer during the snow-melting process is studied theoretically and experimentally in the present work, which can be taken as references for system design and large-scale applications in future. Main conclusions are summarized as follows:1) Characteristics of the pavement temperature fieldA two-dimensional transient heat conductive model is presented and boundary conditions among the pavement, snow layers and the ambient are considered. Using Brian ADI algorithm, the model is solved numerically and a snow-melting simulation program is developed based on the compiler of Visual Basic. This program can output some important results including the pavement temperature field, the height of snow layers, the height of liquid film and the idling characteristic curve.For the solar radiation model, the error of Wu-model during the expansion of Fourier series is modified. Based on Mallat algorithm and Daubechies-type wavelets, a new wavelet transform model is presented for continuous solar radiation. Simulated results show that the model has a better analysis capacity on both time domain and frequency domain, and can also be used in other solar photo-thermal applications.2) Numerical simulation of road snow-melting processThe whole snow-melting process, including the idling process, the snow-melting process and the after-snow process, is simulated. For the idling process, some terms including the zero boundary and the zero time are defined, and a wave-shaped model of the pavement temperature field is presented, which takes the decaying effect of the fluid temperature into account. A general idling characteristic curve and an improved idling model, linking the idling load, the idling time, the snowfall velocity and snow-melting classes, are presented.For the snow-melting process, the types and characteristics of snow layers are analyzed, and the heat and mass transfer model among the pavement, snow layers and the ambient is established based on the capillary effect of porous media. Dynamic and static simulated results under different conditions show that, compared with the heating temperature, the layout of embedded pipes has more obvious effects on the snow-melting process. Besides, a proper idling period can effectively reduce the snow-melting time. For the after-snow process, a heat and mass transfer model for the evaporation of the liquid film on pavements is established. Simulated results show that the order of weather parameters affecting the evaporation is as follows: solar radiation > ambient temperature > wind speed > relative humidity. An improved heating mode is also presented, which is useful for energy saving.3) Experimental investigation of a snow-melting systemA small-scale snow-melting experimental system is built successfully. Heat conductivity of concrete cement pavement is measured using the direct-heating method, and the effects of moisture content on the heat transfer are analyzed.Through the test on physical properties of snow and ice, the free-area ratio, the snow-melting velocity and the snow-melting time, the melting behavior of snow and ice under different weather conditions are obtained and compared with simulated results. For the free-area ratio curve, an image processing method based on the Photoshop software is used. Experimental results show that the density of crushed ice, artificial snow and natural snow at 0oC is about 605～690kg/m3, 225kg/m3, 125kg/m3, respectively.The free-area ratio curve can be divided into three stages: initial stage, slow stage and rapid stage. For ice and snow layer, the critical free-area ratio is different. During the ice-melting process, the air-layer phenomenon between the road surface and the bottom of ice layer may occur, which may prolong the melting time to some extent. In contrast, the above phenomenon usually doesn't happen during the snow-melting process. With regard to the average melting velocity (g/min), the order is as follows: crushed ice > solid ice > artificial snow > natural snow. For the same thickness, however, the order of the melting time is as follows: natural snow > artificial snow > crushed ice > solid ice.4)Model of physical properties of snowA simplified model for the capillary height in snow is built and suitable within the range of 100～400kg/m3. A fractal model is presented to describe the random growth process of fresh snow. The intrinsic relationship between the movement randomness and porous structure is revealed. Besides, a new algorithm on the heat conductivity varying with the density is obtained, which has a good accuracy within the range of 100～150kg/m3.