The thermal instability caused by the variable physical property of the gas flow in tubeis investigated theoretically. Critical phenomena is found that when the heat flux incresingapproaches its critical value, the outlet temperature increases rapidly. The reason is thatthe increasing of the viscosity and decreasing of the density of the gas while the temperatureincreases cause the more rapidly increasing of the flow drag. The critical heat fiux of thephemonena is affected by the pressure drop, fluid property, dimension of the tube.
