Fluid-heating device

1. A vehicle fluid-heating device for heating fluid comprising:

a heater unit configured to have a heater and a heating portion, the heating portion being formed so as to cover a surrounding of an outer surface of the heater; and

a tank configured to accommodate the heater unit, wherein the heating portion includes

an inner heat exchange surface formed on an inner surface of a through hole penetrating through an inner side of the heater, the inner heat exchange surface being configured to perform heat exchange with the fluid, and

an outer heat exchange surface formed on an outer-wall portion of an outer side of the heater, the outer heat exchange surface being configured to perform heat exchange with the fluid,

a heat transfer area of the outer-wall portion is larger than a heat transfer area of the through hole,

the outer-wall portion has outer circumferential fins, the outer circumferential fins being configured to project towards an outer circumference,

the tank has a top surface, the top surface being configured to close an opening portion for inserting the heater unit,

the outer circumferential fins are provided so as to extend substantially parallel to the top surface,

the through hole forms an inner circumferential flow channel, the inner circumferential flow channel being configured such that the fluid flows therethrough,

the outer-wall portion providing an outer circumferential flow channel, the outer circumferential flow channel being configured such that the fluid flows between the outer-wall portion and an inner wall of the tank,

the inner circumferential and the outer circumferential flow channels being configured to permit fluid to flow from the inner circumferential flow channel to the outer circumferential flow channel continuously, and

a flow channel area of the outer circumferential flow channel is larger than a flow channel area of the inner circumferential flow channel.

2. The vehicle fluid-heating device according to claim 1 , wherein

the top surface is connected with the heating portion.

3. The vehicle fluid-heating device according to claim 2 , wherein

the heater unit has

a connecting portion configured to connect the top surface and the heating portion, and

a heat releasing portion formed so as to project out from the connecting portion.

4. The vehicle fluid-heating device according to claim 3 , wherein the connecting portion is formed such that a sectional area of a first portion connected to the heating portion is smaller than a sectional area of a second portion connected to the top surface.

5. The vehicle fluid-heating device according to claim 2 , wherein

both ends of the heater are connected with the top surface via the heating portion.

6. The vehicle fluid-heating device according to claim 2 , wherein

the heater has a heat generating part, the heat generating part being formed to have a helical-shape,

the through hole is formed so as to have a diameter smaller than an inner circumference of the heat generating part, the through hole being configured to penetrate through the heat generating part along a center axis thereof, and

the outer-wall portion is formed so as to have a diameter larger than an outer circumference of the heat generating part, the outer-wall portion being configured to face the inner wall of the tank.

7. The vehicle fluid-heating device according to claim 2 , wherein

the through hole has inner circumferential fins, the inner circumferential fins being configured to project towards an inner circumference.

8. The vehicle fluid-heating device according to claim 7 , wherein

the inner circumferential fins are formed along a flowing direction of the fluid.

9. The vehicle fluid-heating device according to claim 7 , wherein the outer circumferential fins are provided closer to the top surface than a central portion of the tank and formed such that a distance from a proximal portion is increased towards the top surface as compared to that at the central portion of the tank.

10. The vehicle fluid-heating device according to claim 7 , wherein the outer circumferential fins are provided farther from the top surface than a central portion of the tank and formed such that the distance from a proximal portion is increased with increasing distance from the top surface as compared to that at the central portion of the tank.

11. The vehicle fluid-heating device according to claim 7 , wherein

the inner circumferential fins are formed so as to extend towards the inner circumference in a radiating pattern.

12. The vehicle fluid-heating device according to claim 7 , wherein

the outer circumferential fins are formed along a flowing direction of the fluid.

13. The vehicle fluid-heating device according to claim 7 , wherein

the outer-wall portion of a proximal portion of the outer circumferential fins is adjacent to the heater.

14. The vehicle fluid-heating device according to claim 7 , wherein

a number of the outer circumferential fins is greater than a number of the inner circumferential fins.

15. The vehicle fluid-heating device according to claim 7 , wherein

a length of the outer circumferential fins is greater than a length of the inner circumferential fins.

16. The vehicle fluid-heating device according to claim 1 , wherein the tank has

a supply port configured to supply the fluid, and

a discharge port configured to discharge the fluid, and

the discharge port opens so as to be aligned with the supply port on a same surface of the tank.

17. The vehicle fluid-heating device according to claim 16 , wherein

one of the supply port and the discharge port opens on a line extending from the through hole.

18. The vehicle fluid-heating device according to claim 17 , wherein

the supply port opens on the line extending from the through hole.