1. Method for determining the heat transfer characteristics and the wall thicknesses of a heat conducting body having an internal structure for a flow medium to flow through, comprising: passing through the internal structure of the body a first flow medium having a first temperature; registering a first thermal image of an external surface of the body as a function of time using a pixelised image detector; and using a processor to: process the first thermal image in order to obtain first measured surface temperatures (T S, meas (t,x,z,y)) for a given set of points in time (t) and a given set of points (x,y,z) on the surface; predict first surface temperatures (T S, pred (t,x,y,z)) resulting from the transient flow through the body of the same external surface of the body and for the same set of points in time using a numerical model (M num ); apply a best fit method (fit) to the first measured surface temperatures (T S, meas (t,x,z,y)) and the first predicted surface temperatures (T S, pred (t,x,y,z)) and determining a measure of fit between them; and optimise the first predicted surface temperatures (T S, pred (t,x,y,z)) by means of an iteration until the measure of fit between the first measured surface temperatures (T S, meas (t,x,z,y)) and the first predicted surface temperatures (T S, pred (t,x,y,z)) is optimised; wherein the numerical model (M num ) includes initial values for the heat transfer coefficients (h i ) and wall thicknesses (δ i ) of the internal structure of the body and an estimated two- or three-dimensional geometry model (M geom ) of the internal structure of the body wherein the heat transfer coefficients, the wall thicknesses and the two- or three-dimensional geometry model (M geom ) are optimised in the iteration until the measure of fit between the first predicted temperature values (T S, pred ) and the first measured temperature values (T S,meas (t,x,z,y)) is optimised, and final heat transfer coefficients (h f (x,y,z)) and wall thicknesses (δ f (x,y,z)) and final values for the geometry of the internal structure are determined that correspond to the optimised measure of fit; and the fit between the first measured surface temperatures (T S,meas (t,x,z,y)) and the first predicted surface temperature (T S, pred (t,x,z,y)) is determined according to one or more of the following criteria; the absolute surface temperature at a given pixel; the change of temperature from the initial temperature over time; the local derivative of temperature with respect to time or space; the combined local derivative with respect to time and space.