In a vehicle air conditioning apparatus, the temperature of air to be blown into a passenger compartment of a vehicle is generally controlled by adjusting an air mixing ratio of the volume of cool air to the volume of heat air by an air mixing door. In such an air mixing type air conditioning apparatus, it is proposed to adjust the air mixing ratio by a rotational operation of a heater core without using the air mixing door. For example, the heater core is rotationally moved so as to control the volume of cool air bypassing the heater core. In a maximum cooling operation, the heater core is for example moved to a position where the heater core does not disturb the flow of the cool air.
Also, a compact pipe coupling structure that defines passages through which an internal fluid of a heater core flows and allows the rotational operation of the heater core is proposed in Japanese Patent Publication No. 2006-44636 (US 2006/0005959 A1). As shown in FIG. 7, the pipe coupling structure is constructed of a coaxial double pipe unit 616 including a fixed member 616a and a rotation member 616b. The fixed member 616a is fixed to an air conditioning case 11. The rotation member 616b is coupled to the fixed member 616a, but is rotatable about a rotation axis A. The rotation member 616b is further coupled to a heater core 615 and rotatable with the heater core 615.
The fixed member 616a and the rotation member 616b coaxially form the passages. The internal fluid as a heat source, such as a hot water, flows in and out of the heater core 615 through the passages. The fixed member 616a and the rotation member 616b are sealed by an O-ring 635. For example, the O-ring 635 is interposed between an inner circumferential wall of the fixed member 616a and an outer circumferential wall of the rotation member 616b. Namely, the O-ring 635 is compressed in a direction perpendicular to the rotation axis A of the rotation member 616b. 
In this sealing structure, however, if the heater core 615 is subjected to a load F, an axis of the rotation member 616b is displaced from an axis of the fixed member 616a. At this time, a compression ratio of the O-ring 635 is uneven along its circumference. As a result, the internal fluid is likely to leak at a position where the compression ratio is the smallest.