Case Study: CFD Simulation of a Heat Recovery Steam Generator (HRSG)

Project Overview

This case study presents a comprehensive Computational Fluid Dynamics (CFD) simulation of a Heat Recovery Steam Generator (HRSG) to evaluate exhaust gas flow behavior, pressure losses, and heat transfer performance under realistic operating conditions.

HRSGs are critical components in combined-cycle power plants and cogeneration facilities, where waste heat from gas turbine exhaust is recovered to generate steam for power production or process use. Non-uniform flow distribution, excessive pressure drop, and thermal stratification inside HRSG ducting and heat exchanger modules can significantly reduce system efficiency and increase operational risks.

The objective of this study is to demonstrate how CFD can be used as a design and optimization tool to assess HRSG performance and identify engineering improvements prior to fabrication or retrofit.

Objectives of the Study

The primary objectives of this CFD analysis are to:

• Evaluate exhaust gas flow distribution through the HRSG inlet duct, heat exchanger sections, and outlet stack

• Identify flow non-uniformity, recirculation zones, and stagnation regions

• Quantify pressure drop across HRSG modules and assess impact on turbine back-pressure

• Analyze temperature distribution and thermal stratification

• Evaluate heat transfer effectiveness of economizer, evaporator, and superheater sections

• Provide design insights and optimization recommendations to improve HRSG thermal performance and reliability

System Description

The Heat Recovery Steam Generator considered in this study consists of the following major sections:

• Inlet duct and plenum, guiding exhaust gases from the gas turbine

• Economizer, used to preheat feedwater

• Evaporator, where water is converted to saturated steam

• Superheater, raising steam temperature above saturation

• Outlet duct and stack, discharging cooled exhaust gases

Due to the large scale and complex finned tube geometry, the tube banks are modeled using an equivalent porous media approach, enabling accurate prediction of pressure loss and heat transfer behavior without explicitly resolving individual tubes and fins.

Geometry and Computational Domain

The computational domain includes:

• Full 3D representation of the inlet duct and transition sections

• Porous regions representing economizer, evaporator, and superheater modules

• Outlet stack region up to atmospheric discharge

This approach balances model fidelity and computational efficiency, making it suitable for industrial-scale HRSG simulations.

Material Properties

• Working fluid: high-temperature exhaust gas

• Temperature-dependent thermophysical properties are used

• Heat exchanger regions include volumetric heat sink terms to represent heat extraction by steam/water circuits

Boundary Conditions

• Inlet

  • Velocity inlet with swirl added profile

  • Uniform or profiled temperature corresponding to turbine exhaust conditions

• Outlet

  • Fixed static pressure representing stack discharge

• Walls

  • No-slip velocity condition

  • Adiabatic or specified heat transfer conditions depending on insulation

• Porous Zones

  • Directional permeability and inertial resistance coefficients

  • Volumetric heat source/sink terms

Governing Equations:

The CFD simulation is based on the three-dimensional, steady-state Reynolds-Averaged Navier–Stokes (RANS) equations, coupled with the energy equation for turbulent heat transfer. The equations are discretized using the finite volume method.

Continuity Equation

The conservation of mass for an incompressible or weakly compressible flow is expressed as:

RANS Momentum Conservation Equation

The RANS momentum equation for turbulent flow is given by:

Energy Conservation Equation

The thermal behavior of the HRSG exhaust gas is governed by the energy equation:

Porous Media Modeling for Tube Bundles

The momentum loss in heat exchanger regions is modeled as:

Solver Strategy

• Steady-state solution approach

• Pressure-velocity coupling using iterative correction

• Second-order discretization schemes for convection

• Convergence monitored via residual reduction and stabilization of:

  • Pressure drop

  • Outlet temperature

  • Heat transfer rate

Post-Processing and Key Results

Flow Distribution

Initial simulations revealed a significant "jetting" effect, where the high-velocity exhaust gas concentrated at the center of the first tube bank, leaving stagnant zones at the corners of the duct. This non-uniformity (measured by the RMS velocity deviation) was approximately 25%, exceeding the design target of <10%.

Optimization through Internals

To mitigate this, various configurations of distribution plates and turning vanes were simulated. The CFD results demonstrated that a specific perforated plate porosity (approx. 45-50%) placed in the diffuser section could successfully redistribute the flow.

Heat Transfer Performance

Heat transfer effectiveness is directly linked to flow uniformity. Regions with poor velocity distribution exhibit reduced thermal performance.

Engineering Insights Gained

• Improve inlet duct geometry or add internal elements (vanes)to enhance flow uniformity

• Optimize transition sections to reduce pressure losses

• Calibrate porous media coefficients using vendor or test data

• Improve exhaust gas distribution to maximize heat recovery

Industrial Applications

Combined Cycle Power Plants

• In combined-cycle power plants, HRSGs recover thermal energy from gas turbine exhaust to generate steam for steam turbines. CFD analysis helps in:

  • Optimizing exhaust gas flow distribution

  • Reducing turbine back-pressure caused by excessive HRSG pressure drop

  • Enhancing steam generation efficiency

Cogeneration (CHP) Facilities

• In combined heat and power plants, HRSGs supply steam for both electricity generation and process heating. CFD simulations support:

  • Improved thermal integration between power and process systems

  • Stable steam production under varying operating loads

  • Reduced thermal stress due to uneven gas flow

Process Industries

• Industries such as oil & gas, petrochemical, chemical, and fertilizer plants use HRSGs for waste heat recovery from fired heaters and process furnaces. CFD assists in:

  • Evaluating retrofits and capacity upgrades

  • Identifying maldistribution leading to localized overheating

  • Ensuring compliance with operational safety margins

Steel, Cement, and Metallurgical Plants

• High-temperature exhaust gases from kilns and furnaces are routed through HRSGs to recover energy. CFD is used to:

  • Handle complex particulate-laden flows

  • Minimize fouling and erosion-prone regions

  • Improve long-term operational reliability

Marine and Offshore Applications

HRSGs installed on offshore platforms and large marine vessels benefit from CFD-driven optimization to:

• Reduce footprint and weight

• Ensure robust performance under variable operating conditions

• Improve fuel efficiency and emissions performance

Benefits to Industry

• The application of CFD analysis to HRSG design, operation, and optimization delivers tangible technical and commercial benefits across the asset lifecycle.

  Improved Thermal Efficiency

  • Enhanced heat recovery due to uniform exhaust gas distribution

  • Increased steam generation rates without additional fuel consumption

  Reduced Pressure Loss and Operational Costs

  • Lower gas turbine back-pressure improves overall plant output

  • Reduced fuel consumption and operating expenditure

  Enhanced Equipment Reliability

  • Identification of thermal stratification and hot spots

  • Reduced risk of tube overheating, thermal fatigue, and premature failure

  Design Optimization and Risk Reduction

  • Informed decision-making during design and retrofit stages

  • Fewer physical prototypes and design iterations

  Operational Flexibility

  • Improved HRSG performance under part-load and transient conditions

  • Better response to changing process demands

  Lifecycle Cost Reduction

  • Lower maintenance requirements

  • Extended equipment service life

  • Reduced unplanned shutdowns

  Environmental and Regulatory Benefits

  • Improved energy efficiency reduces greenhouse gas emissions

  • Supports compliance with environmental and sustainability targets