Heat exchanger

1. A heat exchanger comprising:

a heat exchanger core comprising first fluid flow channels and second fluid flow channels for exchange of heat between first fluid in the first fluid flow channels and second fluid in the second fluid flow channels;

a first manifold portion to direct the first fluid between the first fluid flow channels in the heat exchanger core and a first fluid interface portion comprising fewer first fluid flow channels than the heat exchanger core, and

a second manifold portion to direct the second fluid between the second fluid flow channels and a second fluid interface portion comprising fewer second fluid flow channels than the heat exchanger core;

wherein the first manifold portion comprises at least one tunnel portion extending through the second manifold portion at an angle to the direction of second fluid flow through the second manifold portion, the first manifold portion is configured to direct at least part of the first fluid through the inside of the at least one tunnel portion and the second manifold portion is configured to direct the second fluid around the outside of the at least one tunnel portion.

2. The heat exchanger according to claim 1 , wherein the first manifold portion is configured to direct the first fluid around a turn between the first fluid flow channels of the heat exchanger core and the at least one tunnel portion or between the first fluid interface portion and the at least one tunnel portion.

3. The heat exchanger according to claim 2 , wherein said turn comprises a turn of at least 45 degrees.

4. The heat exchanger according to claim 1 , wherein an angle between a direction of first fluid flow through said at least one tunnel portion and a direction of second fluid flow through the second manifold portion is at least 45 degrees.

5. The heat exchanger according to claim 1 , wherein the first fluid interface portion is on an opposite side of the second manifold portion to an entry/exit region of the first fluid flow channels of the heat exchanger core.

6. The heat exchanger according to claim 1 , wherein a leading edge of said at least one tunnel portion in the direction of second fluid flow is shaped to direct the second fluid around the outside of said at least one tunnel portion.

7. The heat exchanger according to claim 1 , wherein a leading edge of said at least one tunnel portion has a round, oval, diamond or aerofoil-shaped cross-section.

8. The heat exchanger according to claim 7 , wherein said at least one tunnel portion has an aerofoil-shaped cross-section and a leading edge of the aerofoil-shaped cross-section points towards one of the second fluid interface portion and the second fluid flow channels of the heat exchanger core.

9. The heat exchanger according to claim 1 , wherein the first manifold portion comprises a plurality of said tunnel portions extending through the second manifold portion.

10. The heat exchanger according to claim 1 , wherein the first manifold portion comprises an outer portion to direct part of the first fluid around the outside of the second manifold portion between the first fluid flow channels of the heat exchanger core and the first fluid interface portion.

11. The heat exchanger according to claim 10 , comprising at least one fin bridging between an outer surface of the second manifold portion and an inner surface of said outer portion of the first manifold portion.

12. The heat exchanger according to claim 1 , comprising at least one separator grid disposed inside the second manifold portion to partition the flow of second fluid flowing through the second manifold portion.

13. The heat exchanger according to claim 12 , wherein at least one of said at least one separator grid is coupled to an outer surface of at least one of said at least one tunnel portion.

14. The heat exchanger according to claim 1 , wherein the heat exchanger comprises an integrated mass of consolidated material.

15. A system comprising:

a combustor to generate heat by combusting a fuel; and

a recuperator to recover heat from the exhaust gas output by the combustor;

wherein the recuperator comprises the heat exchanger according to claim 1 .

16. The system according to claim 15 , wherein the second fluid comprises the exhaust gas and the first fluid comprises air to be heated by the exhaust gas before being supplied to the combustor.

17. A method of manufacturing a heat exchanger comprising:

forming a heat exchanger core comprising first fluid flow channels and second fluid flow channels for exchange of heat between first fluid in the first fluid flow channels and second fluid in the second fluid flow channels;

forming a first manifold portion to direct the first fluid between the first fluid flow channels in the heat exchanger core and a first fluid interface portion comprising fewer first fluid flow channels than the heat exchanger core, and

forming a second manifold portion to direct the second fluid between the second fluid flow channels and a second fluid interface portion comprising fewer second fluid flow channels than the heat exchanger core;

wherein the first manifold portion comprises at least one tunnel portion extending through the second manifold portion at an angle to the direction of second fluid flow through the second manifold portion, the first manifold portion is configured to direct at least part of the first fluid through the inside of the at least one tunnel portion and the second manifold portion is configured to direct the second fluid around the outside of the at least one tunnel portion.

18. The method of claim 17 , wherein the heat exchanger is made by additive manufacture.