Benjamin Kirollos

An Energy-Based Method for Predicting the Additive Effect of Multiple Film Cooling Rows

There have been numerous studies reporting film effectiveness for film rows in isolation, which have led to correlations which are used for preliminary design. Many applications require multiple film cooling rows. Although there is some published data which deals with the combined effect of multiple rows, in most design situations the additive effect is computed using correlations for single rows. The most widely used method is the Sellers superposition method. In many applications the method gives accurate results. Although the method is to some extent physically based, energy is not conserved within the model, and in certain situations this limitation can be shown to lead to an under-prediction of the film effectiveness.In this paper, a new energy-based method for predicting the additive effect of multiple film cooling rows is outlined. The physical basis and limitations of the model are discussed. Predictions conducted using the new method are compared with CFD data and contrasted with the Sellers method. In situations where energy conservation is required to avoid under-prediction of effectiveness the method is shown to be advantageous.Copyright

Reverse-Pass Cooling Systems for Improved Performance

Total heat transfer between a hot and a cold stream of gas across a nonporous conductive wall is greatest when the two streams flow in opposite directions. This counter-current arrangement outperforms the co-current arrangement because the mean driving temperature difference is larger. This simple concept, whilst familiar in the heat exchanger community, has received no discussion in papers concerned with cooling of hot-section gas turbine components (e.g., turbine vanes/blades, combustor liners, afterburners). This is evidenced by the fact that there are numerous operational systems which would be significantly improved by the application of “reverse-pass” cooling. That is, internal coolant flowing substantially in the opposite direction to the mainstream flow. A reverse-pass system differs from a counter-current system in that the cold fluid is also used for film cooling. Such systems can be realized when normal engine design constraints are taken into account. In this paper, the thermal performance of reverse-pass arrangements is assessed using bespoke 2D numerical conjugate heat transfer models, and compared to baseline forward-pass and adiabatic arrangements. It is shown that for a modularized reverse-pass arrangement implemented in a flat plate, significantly less coolant is required to maintain metal temperatures below a specified limit than for the corresponding forward-pass system. The geometry is applicable to combustor liners and afterburners. Characteristically, reverse-pass systems have the benefit of reducing lateral temperature gradients in the wall. The concept is demonstrated by modeling the pressure and suction surfaces of a typical nozzle guide vane with both internal and film cooling. For the same cooling mass flow rate, the reverse-pass system is shown to reduce the peak temperature on the suction side (SS) and reduce lateral temperature gradients on both SS and pressure side (PS). The purpose of this paper is to demonstrate that by introducing concepts familiar in the heat exchanger community, engine hot-section cooling efficiency can be improved whilst respecting conventional manufacturing constraints.

Experimental and CFD Studies of NGV Endwall Cooling

For engines operating at high turbine entry temperatures it is increasingly important to cool the high pressure nozzle guide vane (HP NGV) endwalls. This is particularly so for low NOx combustors operating with flatter outlet temperature distributions.Double-row arrangements of film/ballistic cooling holes upstream of the NGV passage have been employed in production engines. Optimisation of such systems is non-trivial, however, due to the complex nature of the flow in the endwall region. Previous studies have reported that strong cross passage pressure gradients lead to migration of coolant flow and boundary layer flow within the passage. In addition the vane potential field effects lead to non-uniform blowing ratios for holes upstream of the vanes. It has also been reported that inlet total pressure and turbulence profiles have a significant effect on the development of the film cooling layer.In this study, endwall film cooling flows are studied experimentally in a large-scale low-speed cascade tunnel with engine-realistic combustor geometry and turbulence profiles. At very low blowing ratios mild cross-passage migration effects are observed. At higher blowing ratios more realistic of the engine situation no cross-passage migration is observed. This finding is somewhat contrary to the classical view of endwall secondary flow, which is presented as significant at the scale of the vane passage by several authors. The difference arises in part because of the thinning of the boundary layer due to strong acceleration in the vane inlet contraction. The findings are further supported by CFD simulations.Methods of improving conventional double-row systems to offer improved cooling of the endwall are also discussed.Copyright

The experimental static mechanical performance of ironed repaired GFRP–honeycomb sandwich beams

Damaged glass fibre reinforced plastic–honeycomb core sandwich beams are repaired using uncured glass fibre reinforced plastic fabrics and a handheld iron. The effect of iron temperature, application time and pressure on the effectiveness of repair is investigated by measuring the failure load and flexural stiffness of the repaired beams using third span four-point bending tests. Repairs are tested in compression and tension. A repair process is suggested which consistently recovers 95% of the compressive strength and 77% of the tensile strength of the damaged beam. The repair is shown to have little effect on beam flexural stiffness.

Cooling Optimization Theory—Part I: Optimum Wall Temperature, Coolant Exit Temperature, and the Effect of Wall/Film Properties on Performance

Gas turbine cooling system design is constrained by a maximum allowable wall temperature (dictated by the material and the life requirements of the component), minimum coolant mass flow rate (the requirement to minimize cycle-efficiency cost), and uniform wall temperature (to reduce thermal stresses). These three design requirements form the basis of an iterative design process. The relationship between the requirements has received little discussion in the literature, despite being of interest from both a theoretical and a practical viewpoint. In this paper, we consider the optimum cooling system for parts with both internal and film cooling. We show analytically that the coolant mass flow rate is minimized when the wall temperature is uniform and equal to the maximum allowable wall temperature. Thus, we show that achieving uniform wall temperature achieves minimum coolant flow rate, and vice versa. The purpose is to clarify the interplay between two design requirements that are often discussed separately in the literature. The penalty (in terms of coolant mass flow) associated with cooling nonisothermal components is quantified. We show that a typical high pressure nozzle guide vane (HPNGV) operating isothermally at the maximum allowable wall temperature requires two-thirds the coolant of a typical nonisothermal vane. The optimum coolant exit temperature is also considered. It is shown analytically that the optimum coolant exit temperature depends on the balance between the mean adiabatic film cooling effectiveness, the nondimensional mass flow rate, and the Biot number of the thermal barrier coating (TBC). For the large majority of gas turbine cooling systems (e.g., a typical HPNGV) it is shown that the optimum coolant exit temperature is equal to the local wall temperature at the point of injection. For a small minority of systems (e.g., long effusion cooling systems operating at low mass flow rates), it is shown that the coolant exit temperature should be minimized. An approximation relating the wall/film properties, the nondimensional mass flow, and the overall cooling effectiveness is derived. It is used to estimate the effect of Biot number (TBC and metal), heat transfer coefficient (HTC) ratio, and film properties on the performance of a typical HPNGV and effusion cooling system. In Part II, we show that designs which achieve uniform wall temperature have a particular corresponding internal HTC distribution.

Cooling Optimisation Theory: Part 2 — Optimum Internal Heat Transfer Coefficient Distribution

Gas turbine cooling system design is constrained by a maximum allowable wall temperature (dictated by the material, the life requirements of the component and a given stress distribution), the desire to minimise coolant mass flow rate (requirement to minimise cycle-efficiency cost) and the requirement to achieve as close to uniform wall temperature as possible (to reduce thermal gradients, and stress). These three design requirements form the basis of an iterative design process. The relationship between the requirements has received little discussion in the literature, despite being of interest from both a theoretical and a practical viewpoint. In the companion paper, we show analytically that the coolant mass flow rate is minimised when the wall temperature is uniform and equal to the maximum allowable wall temperature. In this paper, we show that designs optimised for uniform wall temperature have a corresponding optimum internal heat transfer coefficient (HTC) distribution. In this paper, analytical expressions for the optimum internal HTC distribution are derived for a number of cooling systems, with and without thermal barrier coating. Most cooling systems can be modelled as a combination of these representative systems. The optimum internal HTC distribution is evaluated for a number of engine-realistic systems: long plate systems (e.g., combustors, afterburners), the suction-side of a high pressure nozzle guide vane, and a radial serpentine cooling passage. For some systems, a uniform wall temperature is unachievable; the coolant penalty associated with this temperature non-uniformity is estimated. A framework for predicting the optimum internal HTC for systems with any distribution of external HTC, wall properties and film effectiveness is outlined.© 2015 ASME

Cooling Optimisation Theory: Part 1 — Optimum Wall Temperature, Coolant Exit Temperature and the Effect of Wall/Film Properties on Performance

Gas turbine cooling system design is constrained by a maximum allowable wall temperature (dictated by the material and the life requirements of the component), minimum coolant mass flow rate (the requirement to minimise cycle-efficiency cost) and uniform wall temperature (to reduce thermal stresses). These three design requirements form the basis of an iterative design process. The relationship between the requirements has received little discussion in the literature, despite being of interest from both a theoretical and a practical viewpoint. In this paper, we consider the optimum cooling system for parts with both internal and film cooling. We show analytically that the coolant mass flow rate is minimised when the wall temperature is uniform and equal to the maximum allowable wall temperature. Thus, we show that achieving uniform wall temperature achieves minimum coolant flow rate, and vice versa. The purpose is to clarify the interplay between two design requirements that are often discussed separately in the literature. The penalty (in terms of coolant mass flow) associated with cooling non-isothermal components is quantified. We show that a typical high pressure nozzle guide vane (HPNGV) operating isothermally at the maximum allowable wall temperature requires two-thirds the coolant of a typical non-isothermal vane.The optimum coolant exit temperature is also considered. It is shown analytically that the optimum coolant exit temperature depends on the balance between the mean adiabatic film cooling effectiveness, the non-dimensional mass flow rate and the Biot number of the thermal barrier coating (TBC). For the large majority of gas turbine cooling systems (e.g., a typical HPNGV) it is shown that the optimum coolant exit temperature is equal to the local wall temperature at the point of injection. For a small minority of systems (e.g., long effusion cooling systems operating at low mass flow rates) it is shown that the coolant exit temperature should be minimised.An approximation relating the wall/film properties, the non-dimensional mass flow, and the overall cooling effectiveness is derived. It is used to estimate the effect of Biot number (TBC and metal), HTC ratio and film properties on the performance of a typical HPNGV and effusion cooling system.In the companion paper, we show that designs which achieve uniform wall temperature have a particular corresponding internal heat transfer coefficient (HTC) distribution.Copyright