cfd simulations of a heat recovery steam generator for the

CFD SIMULATIONS OF A HEAT RECOVERY STEAM GENERATOR FOR THE AID OF POWER PLANT PERSONNEL

Iván F. Galindo-García, Ana K. Vázquez-Barragán and Miguel Rossano-Román Electrical Research Institute of México (IIE), Reforma 113, Cuernavaca, México, 62490

ABSTRACT Computational Fluid Dynamics (CFD) simulations of the gas

flow inside a Heat Recovery Steam Generator (HRSG) are

presented. The CFD model can be employed for numerical

simulation of existing and very different possible operation

situations and for the purpose of solving problems in power

plants operating in working conditions subjected to change

(change of the gas turbine load, the fuel quality, etc.). The

CFD results provide detailed information of local values, as

well as corresponding fields of variables, such as velocity

components, temperature and pressure drop of the gas.

Besides its multiple potential applications in the design

phase of a HRSG, here the simulation tool is intended to be

used by plant operators and associated plant personnel in

charge of the optimal operation of the plant, so that they can

obtain a graphically detailed understanding of the flow

process. CFD offers a graphic display that permits the operator

to “see” the flow inside the HRSG and understand the effect

changes in operational conditions have on the flow. For

instance two cases are presented here. First the characteristics

of the flow at different thermal loads and the associated heat

transfer are examined. Second the effect on the flow of

changing the swirl angle at the HRSG inlet is analyzed.

Model validation was performed comparing simulation

data to power plant data. Even though a relatively good

agreement was obtained, it is clear that model validation is a

difficult task due to the scarce data from commercial utilities.

In this context validation should refer more to agreement in

trends than absolute values. This validation strategy, however,

can allow personnel from the plant to use CFD tools to obtain

a qualitative and intuitive understanding of the flow process in

the plant.

1. INTRODUCTION A HRSG is a key component in combined cycle power

plants. The HRSG extracts energy from the combustion gases

from the gas turbine and transfers thermal energy to the

water/steam that flows inside multiple sections of heat

exchanger tubes (superheater, evaporator and economizer).

The steam produced in the HRSG is supplied to a steam

turbine to generate power.

The analysis of HRSGs has historically largely been

performed using thermodynamic principles related to the

steam path, without much regard to the gas side of the system.

However, the flow distribution of the gas turbine exhaust is an

important design consideration. It is known that the tubes are

more susceptible to corrosion and rupture when the flow

distribution is strongly nonuniform [1]. In consequence careful

attention has to be given to assemble tools that may help to

eliminate these problems.

One of these tools is numerical simulation and CFD

methods provide a potentially accurate and cost effective tool

that can help to analyze the gas side of a HRSG. CFD helps to

obtain a thoroughly understanding of the fluid dynamics of the

gas and to visualize the flow inside the computational domain.

CFD is becoming a critical part of the design process of

different power plant equipment since CFD makes it possible

to evaluate velocity, pressure, temperature, and species

concentration of fluid flow throughout a solution domain.

During the last 20 years CFD has been applied for modeling

many flows of practical relevance, for instance it has been

used to troubleshoot flow, mixing, combustion, and heat

transfer problems [2,3]. In HSRGs CFD might be used when:

adding a new gas turbine (modification, conversion, upgrade),

retrofitting/adding a duct burner, adding an emissions-control

system, burner not performing to specifications, plant

emissions need to be lowered, troubleshoot flow instabilities,

Proceedings of the 2012 20th International Conference on Nuclear Engineering collocated with the

ASME 2012 Power Conference ICONE20-POWER2012

July 30 - August 3, 2012, Anaheim, California, USA

ICONE20-POWER2012-55214

1 Copyright © 2012 by ASME

excessive pressure drop, poor steam performance, loud noises

from the HRSG inlet duct, etc.[4].

In this work CFD simulations of the gas flow inside a

HRSG are presented. In addition to the multiple potential

applications during the design phase of a HRSG, here the CFD

simulations are intended to be used by plant operators and

associated plant personnel in charge of the optimal operation

of the plant, so that they can obtain a graphically detailed and

intuitive understanding of the process in the HRSG. CFD

offers a graphic display of the flow that permits the operator to

practically “see” the flow and understand the effect changes in

operational conditions have on the flow. The CFD model can

be employed for numerical simulation of existing and very

different possible operation situations and for the purpose of

solving problems in power plants operating in working

conditions subjected to change (change of the gas turbine load,

the fuel quality, etc.).

Simulations have been performed in order to demonstrate

that CFD simulations are a viable tool to study the effect some

parameters have on the distribution of the gas flow. For

instance, two case studies are presented here. For the first case

the characteristics of the flow at different loads and the

associated heat transfer are examined. For the second case the

effect on the flow of changing the swirl angle of the flow

coming from the turbine at the HRSG inlet is analyzed.

2. GENERAL DESCRIPTION OF THE HRSG The reference geometry is a triple pressure HRSG of a 379

MW combined cycle power plant (two gas turbines of 122 and

129 MW, and one steam turbine of 128MW). The main

function of the HRSG is to generate steam at three different

pressure levels with the quality and conditions for feeding a

steam turbine (and its auxiliary services). The HRSG is

equipped with several tube banks through which water flows

for heating (economizer), steam generation (evaporator),

overheating (superheater) and reheating (reheater). The

number of rows of tubes and the number of tubes per row are

selected based upon the heat exchanger duty.

The HRSG consist of an inlet duct connected to the gas

turbine exhaust, a duct with a transition from circular to

rectangular cross section, a second duct which increases the

height of the HRSG and directs the gas to the tube banks area

and a long stack exhaust for the gases. The flow entering the

heating surfaces should be uniform for optimal thermal

performance. The walls of the inlet duct are lined with three

layers of insulating materials to avoid heat loss and prevent

damage to duct materials from the impact of high velocity

exhaust gases. The design of the inlet duct should maintain the

high velocity flow in the center of the passage area, but not

allow high velocities to impact the duct walls.

3. CFD MODEL The development of the CFD model has involved the

following stages: 1. Building the geometry, 2. Mesh

generation, 3. CFD simulation and, 4. Post processing and

analysis of results. The simulations in the present work were

done using the general purpose CFD software “ANSYS

FLUENT” [5].

Geometry The solution of any CFD process begins with the

generation of the geometry. Technical drawings of the

reference HRSG have been consulted to generate the

computational 3D geometry that represents the actual

equipment as closely as possible. The geometry includes the

gas path from the inlet of the HRSG through the HRSG

exhaust stack, i.e. the inlet duct, heat exchanger modules, and

the stack. The geometry is shown in Fig. 1 and some of the

principal dimensions are given in Table 1.

FIG. 1. GEOMETRY OF THE HRSG.

TABLE 1. PRINCIPAL DIMENSIONS OF THE HRSG. Parameter Dimension

Inlet diameter 3.54 m

Duct length 20 m

Heat exchangers height 19.2 m

Heat exchangers cross section 10.45 x 6 m

Exhaust Stack height 45 m

Exhaust Stack diameter 6.24 m

Meshing The accuracy of any result of a CFD simulation depends

on the quality of the mesh. Numerical error is a combination of

many aspects, for example the grid density, discretization

method, and convergence errors [6]. Numerical error can be

minimized using denser grids, higher order discretization

methods and suitable time step size. The limitation for these

factors is computation time, in particular care is to be taken

while generating the mesh since the size of the mesh generated

should be computationally manageable, as time required to get

a converged solution for a CFD problem depends directly on

the size of the mesh. However in all CFD computations results

should be ensured to be grid independent. Generally, it is


important to find an optimum between acceptable results and

computational time.

In this work a mesh sensitivity analysis was performed in

order to evaluate the effect of the mesh on calculations.

Beginning with a coarse mesh, simulations have been carried

out for different sizes meshes. Target quantities (temperature

and velocity) have been obtained as a function of the grid

density. It should be demonstrated that the final result of the

calculations is independent of the grid that is used. This is

usually done by comparing results of calculations on grids with

different grid sizes. Fig. 2 shows the velocity obtained in a

horizontal line crossing the HRSG at a height of 4m using

grids of different level of refinement.

FIG. 2. VELOCITY FOR THREE DIFFERENT MESH SIZES

The computational mesh adopted for these calculations consist

of tetrahedral and hexahedral elements and has approximately

2.7 million elements of unequal size. The regions close to the

inlet and the entrance to the stack were assigned a denser

mesh. Figure 3 shows the mesh at the inlet duct region.

FIG. 3. MESH OF INLET DUCT.

CFD simulation Once the mesh has been generated, appropriate boundary

conditions need to be applied for the surfaces. This step

includes defining the inlet, outlet and walls and specifying the

zones for the heat exchangers. This completes the

preprocessing stage and the problem is ready to be solved in a

CFD solver.

A 3D steady-state, incompressible solution of the

Reynolds-averaged form of the Navier-Stokes equations,

considering the conservation of mass, momentum, and energy

was obtained using Fluent. A standard two equation turbulence

model (k-ε) with wall functions was applied to model the

turbulent flow. Pressure velocity coupling of momentum and

continuity equations was obtained using the SIMPLE

algorithm. The gas was taken as a single component fluid with

density prescribed as a function of temperature, while other

properties are taken to be constant. The governing equations

for the mean flow in tensor notation read [7]:

Continuity:

0

x

ρU

t

ρ

j

j

(1)

Momentum:

ii

j

j

it

ji

i Sx

U

x

Uμμ

xx

p

Dt

DUρ

(2)

Energy:

h

jt

t

jjj

jS

x

H

σ

μ

x

Tλ

xx

HρU

t

P

t

ρH

(3)

The terms Si and Sh are source terms for the momentum and

energy balances respectively.

Heat exchanger modeling The heat exchangers normally used in HRSGs are tube-

type heat exchangers with fins. It is not feasible to model each

tube individually, as this would result in a very large and

complicated computational mesh. Instead, a porous media

approach is adopted where the gas side pressure drop and heat

transfer characteristics are taken into account. The porous

media model adds two source terms to the momentum

equations (Eq. 2): a viscous term and an inertial loss term,

which depend on the molecular viscosity and the square of

velocity, respectively.

i2ii UUρ

2

1CU

α

μS (4)

where coefficients and C2 represent the permeability and the

inertial resistance factor, respectively.


The tube bundle modules are grouped into four separate

modules as shown in Fig. 4 and Table 2. This allows the user

to specify the absorbed heat and pressure drop characteristics

separately for each of the module sections.

FIG. 4. HEAT EXCHANGER MODULES OF THE HRSG.

For the treatment of the heat transfer inside the porous zone,

the energy equation (Eq. 3) is modified in the heat conduction

term, using an effective thermal conductivity, λeff that takes into

account the fluid, λf and solid, λs conductivities and the

porosity, β of the medium.

sλβ1βλλ feff (5)

The porosity is the volume fraction of fluid within the porous

region (i.e., the open volume fraction of the medium). The

formula for porosity factor is,

FcS4S

πD1β

LT

2o (6)

where Do is heat exchanger tube diameter, ST is the transversal

length (pitch), and SL is the axial length. Fc is a factor to take

into account the volume of the fins for the tubes.

Total heat absorbed in each heat exchanger is modeled by

adding an energy source term to the energy equation, Sh in Eq.

3. The value of the source term is calculated based on the

percentage of heat absorbed in each heat exchanger. The

values for absorbed heat and pressure drop are obtained from

the plant’s thermal performance sheets. In Table 2 the

percentage of heat absorbed in each heat exchanger is shown.

TABLE 2. HEAT AND POROSITY PARAMETERS FOR THE HEAT EXCHANGERS.

Module Heat exchangers

Tubes Diam

(m) Heat

Absorbed %

Porosity

1 HP primary superheater (HPSH1) 136 0.038 4.7 0.793

Reheater (RHTR1) 204 0.044 5.5 0.706

HP secondary superheater (HPSH2) 136 0.038 5.7 0.793

Reheater secondary (RHTR2) 204 0.044 8.5 0.706

HP tertiary superheater (HPSH3)

136 0.038 6.4 0.793

2 HP Evaporator (HPEVAP) 680 0.038 24.0 0.664

3 HP primary Economizer (HPECON1)

476 0.038 10.6 0.664

IP superheater (IPSH) 68 0.038 0.7 0.664

IP Evaporator (IPEVAP)

612 0.038 9.4 0.664

LP superheater (LPSH)

68 0.044 0.4 0.631

HP secondary Economizer (HPECON2)

136 0.038 1.1 0.664

4 IP Economizer (IPECON)

68 0.038 1.3 0.664

HP tertiary Economizer (HPECON3)

340 0.038 4.9 0.664

LP Evaporator (LPEVAP)

544 0.038 6.0 0.754

Feed water heater (FWHTR)

544 0.038 10.9 0.664

The rest of the boundary conditions include the HRSG casing

walls, which consist of linear panels, insulation, and the

exterior steel casing. The wall is treated as a perfectly

insulated wall with no heat loss by applying an adiabatically

insulated boundary condition, with no slip boundary condition,

i.e. the gas velocity is set to be zero on the wall. The flow

outlet at the exhaust stack is taken as a standard pressure outlet

boundary condition. Atmospheric pressure is applied at the

exit of the HRSG.

4. MODEL VALIDATION The data needed for model validation, in particular data for

CFD-type calculations, are usually not available in commercial

utilities. As stated in [8], it is impractical and unlikely that

enough experimental data could be collected to provide the

detailed information needed for CFD modeling. Therefore the

global parameters available from the equipment manufacturer

and from routine measurements by plant operators may be

taken as a guide for model validation. In this context validation

refers more to agreement in trends than comparison of absolute

values.


For the validation calculations the HRSG is assumed to be

operating at 100% load. The simulations results are compared

to a few key global design parameters available from the

HRSG design data such as the gas average temperature and

pressure drop at various points inside the HRSG. It should be

noted that the data are taken from thermal performance design

sheets; therefore it is assumed that the reported value is an

average in a plane at that region. Figures 5 and 6 show plots of

gas average temperature and pressure drop, respectively, in

points along the gas path inside the HRSG. The gas

temperature and pressure drop profiles as the gas flows

through the tube banks are almost exact for calculation and

reference data. This almost perfect match is due to the

adequate choices of the coefficients introduced for the porous

media and heat absorbed and it should not be taken as

validation of other flow features, for example the choice of

turbulence modeling. It is acknowledged that more plant data

such as velocity profiles are needed for better analysis.

FIG. 5. AVERAGE GAS TEMPERATURE FOR SIMULATION

AND REFERENCE DATA.

FIG. 6. PRESSURE DROP FOR SIMULATION AND

REFERENCE DATA.

5. TEST CASES In this work two case studies are presented. For the first

case the characteristics of the flow at 100%, 75% and 50%

load and the associated heat transfer are examined. For the

second case the effect on the flow of changing the swirl angle

of the inlet flow is analyzed.

Simulations for different load

Appropriate boundary conditions need to be applied for

the HRSG operating at different loads. Boundary conditions

for these standard design loads were obtained from the plant’s

thermal performance data sheets (however, these boundary

conditions could have been obtained from operating the real

time module to the desired load). The gas turbine exhaust is

the inlet for the HRSG and the exit of the stack is the outlet.

The boundary conditions required include gas flow rates and

temperatures. The outlet boundary was set as a pressure outlet

and the walls were assigned wall boundary conditions. Table 3

shows the boundary conditions for the simulation cases.

TABLE 3.BOUNDARY CONDITIONS FOR DIFFERENT LOAD

Variable 100% 75% 50% Gas Flow Rate kg/s 341 312 262

Average Gas Temperature at HRSG inlet °C

612 539 533.3

Average Gas Temperature at HRSG stack °C

100 101 99

Pressure drop mmWC 300 223.5 157.2

Heat rejected kcal/hr 1.69x108 1.39x10

8 1.10x10

8

Fuel Natural gas

Ambient Temperature °C 19 19 19

Figure 7 shows the gas average temperature in comparison to

design data for 75 and 50 % loads. It may be observed that the

response of the CFD model is a good fit in respect to the real

plant data. The results agree relatively well in practically all

the plotted points. Figure 8 shows a plot comparing the

average gas temperature for the three loads. A very similar

behavior in respect to heat transfer for the three cases can be

observed.


FIG. 7 AVERAGE GAS TEMPERATURE AT 75% AND 50% LOAD FOR SIMULATION AND DESIGN DATA.

FIG. 8. SIMULATION RESULTS OF AVERAGE GAS TEMPERATURE FOR 100, 75 AND 50% LOAD.

Figure 9 shows the velocity contours at a plane at the center of

the HRSG for the three loads. A very similar velocity profile

can be observed, however the lower inlet velocity of the 50%

load case results in lower velocities in general in the HRSG.

FIG. 9. CONTOURS OF GAS VELOCITY ALONG THE

HRSG MID PLANE.

One of the main advantages of CFD simulations is the

visualization capability which can help the analyst or plant

operator to have a better and more intuitive understanding of

the whole process. In particular for the case of operators, who

are used to operate control systems that normally show only

numeric data in a few displays over a flow process diagram

and may not be fluid dynamics experts, the visualization

capability of CFD can help them to gain sensibility to their

actions and see the effects of such actions to the process. For

instance in Fig. 10 three plots show the visualization of the gas

flow for the variables temperature, velocity and pressure.


FIG. 10. VOLUME RENDERINGS FOR TEMPERATURE, VELOCITY AND PRESSURE FOR THE HRSG.

Simulations for different swirl angleThe inlet velocity distribution of the HRSG is specified by

the velocity profile from the exhaust of the gas turbine. The

flow in the HRSG is mainly governed by the swirl angle of the

gas turbine exhaust, which in turn may depend on turbine load,

the geometry of the HRSG, and the equipment installed within

the HRSG, such as flow tube bundles [9].

The HRSG simulated here is designed for an axial exhaust

of the gas turbine. Data for the swirl angle are difficult to

obtain. Some manufacturers even treat this information as

confidential. However, from the published literature, it is

known that swirl angle is function of the gas turbine load and

variation can include change in direction (clockwise and

anticlockwise) depending on the load, Fig 11 [9]. The inlet

velocity distribution consists of a normalized velocity profile.

The HRSG axial velocity considered here is given as a

function of the radius and normalized by the average velocity

as shown in Fig. 12. This velocity profile is applied to the

model using a profile function as given in FLUENT.

FIG. 11. SWIRL ANGLE FOR DIFFERENT LOADS (POSITIVE CLOCKWISE VIEWED FROM DOWNSTREAM) [9].

FIG. 12. NORMALIZED GAS AXIAL VELOCITY AS

FUNCTION OF RADIUS.

The gas turbine will be operated under wide conditions with

very different swirl angles. In this tests swirl angles of -20, 0,

and 20 deg corresponding approximately to gas turbine loads

of 70, 90, and 100%, respectively, have been chosen to

investigate flow and mixing.


Figure 13 shows the flow streamlines for the three different

swirl angles tested and Fig. 14 shows velocity vectors in a

plane in the duct area. In these figures the different direction in

swirl can be observed. The effect of swirl angle on temperature

can be observed in Fig. 15 where temperature contours are

presented at a plane downstream the first module of heat

exchangers. It can be observed that the higher temperature

zones are located depending on the swirl angle at the inlet, a

result that can be very useful for optimization of the plant

operation.

FIG. 13. STREAMLINES FOR CASES WITH DIFFERENT

SWIRL ANGLE.

FIG. 14. VECTOR PLOTS FOR DIFFERENT SWIRL ANGLE.

FIG. 15. CONTOURS OF TEMPERATURE AT A PLANE AFTER THE FIRST MODULE OF HEAT EXCHANGERS.

6. CONCLUDING REMARKS Computations of a HRSG have been undertaken using a

commercial CFD code that solves the 3D-equations for mass,

momentum, and energy in combination with models for

turbulence (standard high-Reynolds-number k-ε model). The

aim was to evaluate a CFD model to investigate flow,

temperature and pressure distributions within a HRSG that can

help plant operators and associated plant personnel to analyze

the HRSG operation for different conditions, for example

operation at different loads or the effect of a change in the inlet

swirl angle. A particular benefit of CFD is the graphic

representation of the flow which can help the analyst or plant

operator to have a better understanding of the gas flow path

and heat transfer characteristics. The CFD model will be part

of an integral simulation tool which includes a real time

lumped-parameter module of the HRSG and all related

systems and controllers to help the user to dynamically

simulate different operational conditions and establish a

particular condition to be simulated with the CFD code.

The model presented here has been evaluated comparing

simulation data to power plant data. Even though a relatively

good agreement in pressure drop and temperature has been

obtained, model validation is a difficult task due to the scarce

data from commercial utilities. In this context validation

should refer more to agreement in trends than absolute values.

This qualitative validation strategy, however, can allow

personnel from the plant to benefit from the use of CFD tools

to obtain a qualitative and intuitive understanding of the flow

process in the plant.


ACKNOWLEDGMENTS Financial support for this work was provided by CFE (the

Mexican utility, Laboratorio de Pruebas a Equipos y

Materiales, LAPEM), and IIE (Electrical Research Institute of

México).

NOMENCLATURE Roman letters

C2 inertial resistance factor (1/m)

Do tube diameter (m)

Fc factor for the porosity of fins of tubes (-)

H specific enthalpy (j/kg)

Ρ pressure (Pa)

T temperature (K)

Ui velocity components (m/s)

k turbulent kinetic energy (m2/ s

2)

Si momentum source term (N/m3)

Sh energy source term (W/m3)

ST transversal length of heat exchanger (m)

SL axial length of heat exchanger (m)

xi coordinates direction (m)

Greek letters

α permeability (m2)

β porosity factor (-)

ε rate of viscous dissipation (m2/ s

3)

λ thermal conductivity (j/s m K)

λeff effective thermal conductivity (j/s m K)

λf fluid phase thermal conductivity (j/s m K)

λs solid phase thermal conductivity (j/s m K)

µ dynamic viscosity (kg/ms)

µt eddy viscosity (kg/ms)

σt turbulent Prandtl number (-)

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[6] Ferziger, J.H., and Peric, M., Computational methods for

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cfd simulations of a heat recovery steam generator for the

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