CHEMICAL ENGINEERING TRANSACTIONS  
 

VOL. 61, 2017 

A publication of 

 

The Italian Association 
of Chemical Engineering 
Online at www.aidic.it/cet 

Guest Editors: Petar S Varbanov, Rongxin Su, Hon Loong Lam, Xia Liu, Jiří J Klemeš 
Copyright © 2017, AIDIC Servizi S.r.l. 

ISBN 978-88-95608-51-8; ISSN 2283-9216 

Development of a Heat Exchanger for Low Pressure Ratio 

Gas Turbines with the Use of CFD Computations and 

Thermodynamic Cycle Analysis 

Christina Salpingidoua, Dimitrios Misirlisb,*, Zinon Vlahostergiosa, Michael Flourosc, 

Stefan Donnerhackc, Kyros Yakinthosa 

aAristotle University of Thessaloniki, Greece  
bTechnological Educational Institute (TEI) of Central Macedonia, Serres, Greece 
cMTU Aero Engines AG, Munich, Germany  

 misirlis@eng.auth.gr 

The performance optimization of energy systems is a field of increased research interest for reasons strongly 

related with economy and environmental sustainability. In this context, a significant amount of effort is 

provided in the direction of gas turbines performance enhancement targeting the reduction of fuel 

consumption and pollutants emissions. A significant contributor in this attempt can be found in the use of heat 

exchangers in recuperative gas turbine configurations which can exploit a significant part of the, otherwise 

wasted, hot-gas thermal energy content in order to preheat the compressor discharge air before the 

combustor, thus reducing fuel requirements and pollutants emissions. For the proper operation of the 

recuperative gas turbine, the integrated heat exchanger should be carefully designed in order to provide a 

favourable combination of sufficiently high thermal effectiveness with tolerable pressure losses. In addition, 

when gas turbines for propulsion applications are considered such as in the case of heli engines, heat 

exchanger weight and dimensions should also be taken into strong consideration. Such an effort is presented 

in the current work, where a heat exchanger for heli engine applications was designed and assessed. At the 

first part of this work, a detailed thermodynamic cycle analysis was performed on a low pressure ratio gas 

turbine configuration, suitable for heli engine applications, the operational conditions of which were based on 

open-literature available data. At the next stage, a tubular heat exchanger was designed taking into 

consideration geometrical constraints and heat exchanger operational conditions while for the heat exchanger 

sizing, literature based heat transfer correlations were applied. The heat exchanger performance was further 

assessed through the use of detailed computations on a CFD model of the integrated heat exchanger in the 

heli engine configuration, based on which the heat exchanger pressure loss and heat transfer characteristics 

were computed. At the final stage of the present work, the heat exchanger effect was assessed on a 

thermodynamic cycle analysis model of the recuperative heli engine, in which the previously derived heat 

exchanger performance characteristics were included. The analysis showed that the use of a heat exchanger 

can provide 10.6 % improvement regarding cycle thermal efficiency and 9.3 % fuel consumption reduction in 

the heli engine gas turbine configuration in relation to a non-recuperative heli engine of the same technological 

level showing the significant optimization potential of this technology targeting the achievement of more 

environmentally friendly and cost-efficient gas turbines for heli engines. 

1. Introduction 

The optimization of the performance of energy systems has always been in the focal point of engineering 

applications and research activities targeting environmental and economic sustainability. The latter has 

become particularly important nowadays due to the increased awareness regarding pollutants emissions and 

the fluctuating fuel prices. A large part of these research activities have been focused on gas turbine 

applications for power plant stations, as presented in Yong et al. (2016), aero engine applications, as 

presented in Schonenborn et al. (2004) or both gas turbines and aero engines, as presented in Salpingidou et 

                               
 
 

 

 
   

                                                  
DOI: 10.3303/CET1761170

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Please cite this article as: Salpingidou C., Misirlis D., Vlahostergios Z., Flouros M., Donnerhack S., Yakinthos K., 2017, Development of a heat 
exchanger for low pressure ratio gas turbines with the use of cfd computations and thermodynamic cycle analysis, Chemical Engineering 
Transactions, 61, 1033-1038  DOI:10.3303/CET1761170  

1033



al. (2016). For such applications the integration of heat exchangers in recuperative gas turbines configurations 

provides a promising technology concept through which both fuel consumption and pollutants emissions can 

be reduced. In this concept, a heat exchanger is installed right after the gas turbine low-pressure turbine since 

a significant part of the hot-gas waste heat can be exploited so as to preheat the compressor discharge air 

right before the latter enters the combustor, hence reducing fuel requirements and pollutants emissions. The 

degree of waste heat exploitation is mainly dependent on the heat exchanger surface with an increase of the 

latter leading to increased heat exchanger thermal effectiveness values and waste heat exploitation. On the 

other hand, the use of increased heat exchange surface values can significantly affect the performance of the 

low-pressure turbine since the imposed pressure losses on the gas-flow can alter the achieved turbine work. 

These imposed pressure losses are also strongly dependent on the heat exchanger core geometry and the 

selection of a non-aerodynamically optimal design can further deteriorate the low-pressure turbine 

performance. As a result for the proper operation of the recuperative gas turbine, the integrated heat 

exchanger should be carefully designed so as to provide a favourable combination of sufficiently high thermal 

effectiveness with tolerable pressure losses. The latter becomes particularly important when gas turbines for 

propulsion applications are considered, such as in the case of heli engines, where heat exchanger weight and 

dimensions should be taken into strong consideration due to the limited available space for installation and the 

strong effect that the heat exchanger own weight can have in the overall benefit of this technology since the 

heat exchanger weight corresponds to a non-detrimental percentage of the overall recuperative heli engine 

weight.  

Taking these parameters under consideration, high-fidelity tools can be utilized for the proper design and 

performance assessment of a heat exchanger for heli engines. These tools can include computational fluid 

dynamics models in order to properly capture the heat transfer and fluid behaviour of the heat exchanger, 

regarding pressure losses and thermal effectiveness, combined with thermodynamic cycle analysis, in order to  

assess the effect of the heat exchanger waste heat recovery on the overall performance of the heli engine. 

Such an effort is presented in the current work, where a heat exchanger for a typical heli engine application 

was designed and assessed. At the first part, a detailed thermodynamic cycle analysis was performed on a 

low pressure ratio gas turbine configuration, suitable for heli engine applications, the operational conditions of 

which were selected based on similar applications presented in open-literature, i.e. Shapiro and Levy (1990). 

At the next step, a tubular heat exchanger was designed taking into consideration the installation geometrical 

constraints and the operational conditions of the heli engine. Furthermore, for the heat exchanger sizing, 

literature based heat transfer correlations were initially applied. Then, the heat exchanger performance was 

assessed through the use of detailed computations on a detailed CFD model of the integrated heat exchanger 

in the heli engine configuration, based on which the imposed heat exchanger pressure loss and achieved heat 

transfer effectiveness were computed.  

At the final stage of the present work, the effect of the heat exchanger waste heat recovery on the heli engine 

performance was assessed on a thermodynamic cycle analysis model of the recuperative heli engine, in which 

the previously derived heat exchanger performance characteristics were included through the use of 

appropriate pressure loss and thermal effectiveness coefficients, following an approach which has been 

successfully applied for recuperative turbofan aero engines as presented in the work of Goulas et al. (2014). 

The analysis showed that the use of a heat exchanger can provide significant improvement in the heli engine 

gas turbine configuration, leading to noticeable fuel consumption and pollutants emissions reduction.  

To the authors’ knowledge, this is the first time that a heat exchanger specifically designed for a heli engine 

application is designed, modelled and computationally assessed regarding flow field development, heat 

transfer and thermodynamic performance taking also into consideration heli engine geometrical constraints. 

As a result, the quantification of the benefits of heli engine recuperative cycle shows the strong potential of this 

technology which will be further investigated in future studies. 

2. Heat exchanger basic design 

The heat exchanger design aimed at a 0.4 thermal effectiveness value in order to provide a design which 

would be compatible with the geometrical constraints of a heli engine and also provide manageable weight 

increase and pressure losses. The used correlations were based on the suggestions of Cengel and Boles 

(2015) and correspond to heat transfer correlations for flows through tube bundles based on circular tubes. 

Due to the fact that these correlations are based on experimental measurements in which a non-negligible 

experimental error is well known to be included, the calculated required heat exchange surface was increased 

by a factor of 20 %.For the sizing of the tubes dimensions and arrangement, a 0-D tool was developed in a 

spread-sheet through which the heat exchanger basic dimensions (tubes diameters, length, staggered 

arrangement and tubes number) were defined. For the estimation of the necessary thermo physical properties, 

input from Shapiro et al. (1990) were used for the heli engine thermodynamic cycle conditions and input from 

1034



Nkoi et al. (2013) were used for the components performance, in a thermodynamic cycle analysis with the use 

of the Gas Turb11 (Kurzke, 2011) gas turbine analysis software, to derive the appropriate reference 

temperature and pressure conditions for the heat transfer correlations.   

The derived heat exchanger design is presented in Figure 1. As it can be seen, a disk-shaped annular wrap-

around cross-flow heat exchanger concept was followed following the suggestions of McDonald (2003) since 

this approach provides good aerodynamic gas flow paths resulting in reduced pressure losses and limited 

need for external ducting and thermal expansion devices. In addition, the basic dimensions of the designed 

heat exchanger were respecting the heli engine installation geometrical constraints (e.g. maximum and 

minimum radius, heat exchanger axial length etc.). In this concept the compressor discharge air is circulating 

inside the circular tubes in the radial direction, while the turbine outlet hot-gas flow is flowing around the 

circular tubes, as it is presented in detail in Figure 1. This concept was further refined through the proper 

design of the staggered tube arrangement core, which consisted of two banks of different tubes arrangement 

i.e. 3/3/3 and 4/4/4, while a two-pass cross flow configuration was selected in order to further enhance the 

achieved heat transfer between hot-gas flow and compressor discharge air.  

    

 
 

  

 
 

          
         

 

           

Figure 1: Heat exchanger design for the heli engine perspective view and enlarged views in the heat exchanger 

core (flow currents are indicated with different colours) 

3. CFD modeling and results 

At the next step, a detailed CFD model consisting of 5.5·106 computational points, corresponding to a slice of 

the heat exchanger design, was created which is presented in Figure 2. The CFD model included three 

different domains, the first one corresponding to the compressor discharge air flow inside the circular tubes, 

the second one corresponding to the solid tube walls and the third one corresponding to the hot-gas flow 

around the circular tubes, as presented in Figure 3. The CFD computations were performed in Ansys CFX 

CFD software (2015), with the use of the Shear Stress Transport (SST) low-Reynolds turbulence model of 

Menter (1994). The selection of SST turbulence model for the CFD computations was based on the 

conclusions of the work of Missirlis et al. (2011) in which the SST turbulence model provided the best results 

in comparison to experimental measurements for a tubular heat exchanger of elliptic profiles for recuperation 

application. Since the SST turbulence model belongs to the low-Reynolds number family, special care was 

provided during the creation of the computational grid in order to ensure an appropriate grid refinement near 

the tube walls, so that the computed y+ value there was in the range of 1-3. In this direction, previously 

acquired knowledge of the necessary grid refinement values from Missirlis et al. (2011) was included during 

the grid creation process in order to place the first computational cell from the walls properly (a normal 

distance of approximately 10-2 mm was used). 

1035



 
 

 
 

 

Figure 2: Computational grid for the heat exchanger CFD model with enlarged view near the tubes core  

The computational nodes were created following an hexaedral philosophy with the use of various 

computational domains (all of which combined created the overall computational domain) in order to arrange 

the computational cells in a way that the corresponding cell angles would not create skewed cells (the vast 

majority of cell angles was ranging from 75 ο to 90 ο while the cells expansion ratio was always kept below 

1.2). The transport equations of the SST model are presented in equations 1 and 2, which are solved together 

with the momentum and energy equations (not presented here due to space limitation reasons).  

𝜕(𝜌𝑘)

𝜕𝑡
+
𝜕(𝜌𝑈𝑗𝑘)

𝜕𝑥𝑗
=

𝜕

𝜕𝜒𝑗
[(𝜇 +

𝜇𝑡
𝜎𝜅
)
𝜕𝑘

𝜕𝑥𝑗
]+𝜇𝑡𝑆

2 − 𝛽′𝜌𝑘𝜔 (1) 

𝜕(𝜌𝜔)

𝜕𝑡
+
𝜕(𝜌𝑈𝑗𝜔)

𝜕𝑥𝑗
=

𝜕

𝜕𝜒𝑗
[(𝜇 +

𝜇𝑡
𝜎𝜔

)
𝜕𝜔

𝜕𝑥𝑗
]+𝛼3

𝜔

𝑘
𝜇𝑡𝑆

2 − 𝛽3𝜌𝜔
2 + (1 − 𝐹1)2𝜌

1

𝜎𝜔2𝜔

𝜕𝑘

𝜕𝑥𝑗

𝜕𝜔

𝜕𝑥𝑗
 (2) 

where 𝐹1 and 𝐹2 are blending functions of the wall distance and the flow variables, 𝜔 is the specific turbulence 

dissipation, 𝜀 is the turbulent dissipation and 𝜇𝑡 is the eddy viscosity. Additional information for the blending 

functions regarding their formulation functions can be found in ANSYS CFX Modeling Guide (2015). For the 

CFD computations for all the transported variables a high resolution advection scheme was adopted which 

was based on the study of Barth and Jespersen (1989). Regarding the thermo physical properties (density, 

thermal capacity, dynamic viscosity, thermal conductivity) which were applied in the CFD computations, 

temperature dependent values were applied in all domains using data of Cengel and Boles (2015) and data 

from international open literature.  

 
        

Figure 3: View of the three computational domains arrangement inside the heat exchanger core (red: cold-air 

flow, grey: solid tube walls, blue: hot-gas flow) 

Regarding the applied boundary conditions, the mass flow was prescribed in both fluid domain inlets together 

with the static temperature and turbulence intensity values, while static pressure was imposed in both fluid 

domains outlets. Rotational periodicity conditions were applied in order to exploit the periodic nature of the 

heat exchanger geometry and reduce its necessary computational size. Moreover, the applied boundary 

condition values (mass flow, pressure, temperature) were derived from the heli engine thermodynamic cycle 

conditions. Typical views of the velocity and temperature distribution inside the heat exchanger are presented 

in Figures 4a and 4b. As it can be seen, the flow field is being characterized by two main regions, the main 

flow region away from the tube walls and the recirculation regions being developed at the back part of the tube 

walls, especially after the last row tubes. Similar flow patterns are presented also for the temperature field 

distribution. These patterns are in complete accordance with the conclusions presented in Missirlis et al. 

(2011) and have a direct impact on the local heat transfer coefficients of the circular tubes with increased 

values being achieved in the circular tubes ‘leading edges’ and lower values near their ‘trailing edges’ regions. 

1036



 

  

Figure 4: (a) Non-dimensional velocity and temperature distribution inside the heat exchanger core. (b) Non-

dimensional temperature distribution inside the heat exchanger core (axial middle plane) 

The analysis of the CFD computations showed that the heat exchanger provided an achieved thermal 

effectiveness of 0.39, a value very close to the initial evaluation of 0.4. In addition, the heat exchanger design 

resulted in very limited pressure losses in both flow currents since the pressure loss percentage (in relation to 

the inlet pressure value) was 0.1 % and 0.2 % for the cold-air and hot-gas flow currents respectively.  

4. Thermodynamic analysis on heli engine 

The last part of this work was focused on the quantification of the benefits of recuperation with the previously 

presented heat exchanger on a heli engine gas turbine application. The heli engine operational conditions  

and parameters were selected based on similar analyses presented in international literature, and more 

specifically in the work of Shapiro et al. (1990), which are summarized in Table 1. Similar conditions are also 

presented in the work of Nkoi et al. (2013). 

Table 1: Heli engine parameters. 

Parameter  Value 

Compressor Efficiency 0.78 

Power Turbine Efficiency 0.9 

Compressor Pressure Ratio 

Heat exchanger cold-air pressure losses 

Heat exchanger hot-gas pressure losses 

Turbine Inlet Temperature (TIT) 

HEX Effectiveness 

Engine airflow 

11.2:1 

0.1 % 

0.2 % 

1,400 K 

0.39 

5 kg/s 

 

 

Figure 5: The heli engine thermodynamic cycle in Gas turb11 

1037



At the next step a 0-D thermodynamic cycle model of the heli engine was set in GasTurb 11 gas turbine 

analysis software, presented in Figure 5, in which the previously derived operational characteristics of the 

designed heat exchanger (i.e. inner/outer pressure losses and heat exchanger effectiveness) were 

incorporated together with the cycle components main parameter (e.g. compressor and turbines efficiencies 

etc.) and a performance analysis was performed. The comparison of the recuperative heli engine results with 

the ones of a non-recuperative heli engine of the same technological level showed a ~10.6 % improvement 

regarding cycle thermal efficiency and a more than 9.3 % fuel consumption reduction. These results clearly 

reflect the significant optimization potential of heat exchanger recuperation in order to achieve fuel 

consumption and pollutants emission reduction, towards to more environmentally friendly and cost efficient 

gas turbines. 

5. Conclusions 

In the present work, a heat exchanger for heli engine applications was designed and assessed. The heat 

exchanger pressure loss and thermal effectiveness characteristics were calculated through the use of detailed 

computations on a CFD model corresponding to a characteristic flow path of the heat exchanger core of the 

integrated heat exchanger in the heli engine configuration. At the final stage, the heat exchanger effect on a 

heli engine was assessed through thermodynamic cycle analysis in relation to a simple, non-recuperative, heli 

engine configuration for similar conditions. The comparison showed that the use of a heat exchanger in a heli 

engine application has a strong potential and can provide significant improvement regarding fuel consumption 

and pollutants emissions reduction in relation to a non-recuperative heli engine, indicating the need for further 

investigation in this direction. 

Acknowledgments  

The authors wish to acknowledge financial support provided by the Research Committee of the Technological 

Education Institute of Central Macedonia, under grant SAT/MC/140115-1/01. 

Christina Salpingidou would like to thank Alexander S. Onassis Public Benefit Foundation for the scholarship. 

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