Hot wire measurements of longitudinal and transverse increments are performed in three different types of flows on a large range of Reynolds numbers (100≲Rλ≲3000). An improved technique based on cumulant expansion of velocity structure functions is used to estimate the spreading of the pdfs and to study their scaling properties in the inertial range. Thus, the rate of intermittency depth through the scales of flow, called here β(Rλ), is experimentally introduced, and it is shown that β(Rλ) has a universal behavior on a very large Reynolds numbers range.

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PurposeThe aim is to study numerically the heat transfer enhancement in a double pipe heat exchanger by using porous fins attached at the external wall of the inner cylinder.Design/methodology/approachThe Brinkman‐Forchheimer extended Darcy model is used in the porous regions. The differential equations subjected to the boundary conditions are solved numerically using the finite volume method. Numerical calculations are performed for a wide range of Darcy number (10−6≤Da≤10−1), porous fins height (0≤Hp≤1) and spacing (0≤Lf≤39) and thermal conductivity ratio (1≤Rk≤100). The effects of these parameters are considered in order to look for the most appropriate properties of the porous fins that allow optimal heat transfer enhancement.FindingsThe results obtained show that the insertion of porous fins may alter substantially the flow pattern depending on their permeability, height and spacing. Concerning the heat transfer effect, it is found that the use of porous fins may enhance the heat transfer in comparison to the fluid case and that the rate of improvement depends on their geometrical and thermo‐physical properties. Performance analysis indicated that more net energy gain may be achieved as the thermal conductivity ratio increases especially at high Darcy numbers and heights.Research limitations/implicationsThe results obtained in this work are valid for double pipe heat exchangers with the same fluid flowing at the same flow rate in the two ducts and for the case of an inner cylinder of negligible thermal resistance.Practical implicationsThe results obtained in this study can be used in the design of heat exchangers.Originality/valueThis study provides an interesting method to improve heat transfer in a double pipe heat exchanger by use of porous fins.

PurposeThe purpose of this paper is to study numerically the fluid flow and heat transfer in an inclined channel provided with heated porous blocks on its lower plate.Design/methodology/approachThe Brinkman‐Forchheimer extended Darcy model with the Boussinesq approximation is adopted for the flow in the porous regions. The governing equations with the appropriate boundary conditions are solved by the control volume method. The effect of some pertinent parameters such as the buoyancy force intensity, the porous blocks shape and height, the porous medium permeability and the Reynolds number are analyzed for various inclination angles ranging from −90° to +90°.FindingsThe results reveal, essentially, that the inclination angle of the channel can alter substantially the fluid flow and heat transfer mechanisms, especially at high Richardson and Darcy numbers. In this case, the maximum and minimum global Nusselt numbers are reached for α=+90° and α=−90°, respectively.Research limitations/implicationsThe results obtained in this work are valid for an inclined channel with porous blocks attached on the heated parts of the lower plate, whereas the upper wall is thermally insulated.Practical implicationsThe results obtained in this worky can be used in the thermal control of electronic components. The use of porous blocks mounted on the heat sources will increase the rate of heat removal in order to maintain the electronic components at an acceptable operating temperature.Originality/valueThe paper provides an interesting method to improve the cooling of electronic devices by use of a porous medium.