Numerical studies for the thermal regime of a high-speed railway tunnel considering piston action on seasonally frozen regions

piston action describes the phenomenon that air at the train nose is pushed forward by the increased pressure and air at the train rear is drawn forward by the decreased pressure when a train passes through a tunnel. The changes of pressure can affect the thermal environment inside the tunnel, and further cause frost damage. In this paper, a fluid-thermal-solid coupled numerical model considering piston action is developed. A high-speed railway tunnel in the northeast of China is taken as an example to explore the temperature distribution laws with computational fluid dynamic (CFD). Afterwards, the effects of air temperature and train velocity on temperature distribution are analyzed. The results show that the piston action can enhance the heat transfer between cold air outside the tunnel and tunnel structure, and can cause more serious frost damage especially at the entrance and exit. The temperature distribution is characterized by three zones, including disturbed zones at two sides of tunnel and undisturbed zone at tunnel middle. The freezing length is closely related to air temperature and train velocity. And also, the lengths are different at vault and rail of tunnel portal, which indicates that the anti-freezing measure should be different at these positions considering the cost. This paper can provide some reference for determining the anti-freezing fortified length of tunnels in cold regions.

Heat island intensities over Brihan Mumbai on a cold winter and hot summer night

A mobile temperature survey of Brihan Mumbai (Greater Bombay) was undertaken on 16 January 1997 which incidentally turned out to be the coldest winter night. Heat islands were found well inside the city, away from the coastal boundary, within a distinct tongue of warm air splitting in three branches roughly along the three sub- urban railway tracks. The finding of the study were in sharp contrast with similar studies conducted during the early seventies by Daniel and Krishnamurthy (1973) and later by Mukherjee and Daniel (1976). They found a remarkable influence of sea on the horizontal temperature distribution in comparison to other factors of urbanisation and noticed the heat island over Malabar Hill, Girgaurn and Cuffe Parade area. The survey conducted after a span of twenty-two years, showed that the temperature distribution in the city and suburbs has been modified significantly and that the effect of urbanisation has overtaken the effect of maritime influence in the formation and maintenance of heat islands. Another mobile temperature survey was conducted during the early hours of 11 May 1997 on the same lines as survey conducted on 16 January 1997, to confirm the findings of earlier survey and to assess seasonal changes in the intensities of heat island. This later survey showed similar pattern of horizontal temperature distribution, though the intensity of heat island observed was only 5.5° C as compared to 11.8° C observed during winter.

Direct observation of three-dimensional transient temperature distribution in SiC Schottky barrier diode under operation by optical-interference contactless thermometry (OICT) imaging

We have developed optical-interference contactless thermometry (OICT) imaging technique to visualize three-dimensional transient temperature distribution in 4H-SiC Schottky barrier diode (SBD) under operation. When a 1 ms forward pulse bias was applied, clear variation of optical interference fringes induced by self-heating and cooling were observed. Thermal diffusion and optical analysis revealed three-dimensional temperature distribution with high spatial (≤ 10 μm) and temporal (≤ 100 μs) resolutions. A hot spot that signals breakdown of the SBD was successfully captured as an anormal interference, which indicated a local heating to a temperature as high as 805 K at the time of failure.