Hengshi (Langfang) Precision Machinery Manufacturing Co., Ltd.

Air leaving a fan, bend, plenum or irregular duct is rarely uniform.

The flow may contain:

• swirl;
• cross-flow;
• large vortices;
• velocity gradients;
• local high-speed regions; and
• unstable flow direction.

These conditions can reduce measurement accuracy, fan efficiency, cooling uniformity and test repeatability.

A honeycomb flow straightener divides the flow into many smaller passages. The walls restrict lateral movement and guide the local velocity toward the passage axis.

The result can be a flow with less swirl and better directional uniformity.

However, a honeycomb does not automatically create laminar flow. That common marketing claim is usually too broad.

Honeycomb Straightening Is Not the Same as Laminarization

Laminar and turbulent flow are determined by more than the presence of a honeycomb.

They depend on factors such as:

• Reynolds number;
• inlet turbulence;
• velocity;
• hydraulic diameter;
• surface condition;
• downstream distance;
• screens; and
• contraction geometry.

A honeycomb is best described as a flow-direction conditioning device.

Its principal function is to suppress transverse velocity components and large-scale rotational motion.

NASA experiments comparing screens, honeycomb and combined configurations found that:

• screens reduced axial turbulence more effectively than lateral turbulence;
• honeycomb reduced lateral turbulence more effectively than axial turbulence; and
• combining the two provided complementary flow-conditioning effects.[1]

This helps explain why high-quality wind tunnels rarely depend on one component alone.

How the Cells Influence the Flow

Imagine a rotating air mass entering one large open duct.

Without internal walls, the air can continue moving both forward and sideways. When the same flow enters hundreds or thousands of narrow cells, lateral movement is restricted by the cell walls.

Inside each cell:

1. the incoming flow is divided;
2. cross-flow interacts with the walls;
3. lateral velocity is reduced;
4. the dominant velocity becomes more closely aligned with the cell axis; and
5. the flow exits as many smaller streams with improved directional consistency.

The individual streams must then mix downstream.

For this reason, the location of the honeycomb relative to screens, contraction sections and the final test area matters.

Why Wind Tunnels Use Honeycomb and Screens Together

A typical low-turbulence wind-tunnel settling chamber may include:

• a protective filter;
• a honeycomb straightener;
• one or more turbulence-reduction screens; and
• a contraction section.

Each component performs a different function.

The honeycomb mainly reduces swirl and transverse motion. Screens help redistribute velocity and reduce certain turbulence components. The contraction accelerates the flow and can further improve relative velocity uniformity.

In one published wind-tunnel design, a fine filter, honeycomb, two turbulence-reduction screens and a 7.3:1 contraction ratio produced flow uniformity within approximately 0.3% of the mean velocity and turbulence intensity around 0.15% in the working region.[2]

These are results from a complete wind-tunnel system. They should not be interpreted as guaranteed performance for an isolated honeycomb.

The Most Important Honeycomb Design Variables

1. Cell Size

Cell size influences:

• lateral flow restriction;
• open area;
• pressure drop;
• structural stability; and
• manufacturing difficulty.

Smaller cells provide more walls across a given area and may improve directional control. However, they may also:

• increase pressure loss;
• increase blockage;
• increase manufacturing cost; and
• become more sensitive to dust or contamination.

The correct cell size depends on the duct dimensions, flow velocity, required uniformity and acceptable resistance.

2. Core Depth

A deeper honeycomb gives the flow more distance to align with the cell axis.

Depth is often considered together with cell diameter or hydraulic diameter as an aspect ratio.

A very shallow core may not provide enough directional control. A very deep core may add unnecessary:

• pressure loss;
• weight;
• cost; and
• installation length.

Longer is not automatically better.

3. Foil Thickness

Foil thickness affects:

• mechanical strength;
• open-area ratio;
• weight;
• welding or brazing stability; and
• resistance to deformation.

Thicker foil may be necessary for large panels, high velocity, vibration or unsupported installations. However, increasing foil thickness also increases blockage.

4. Open-Area Ratio

The open-area ratio describes how much of the frontal area is available for flow.

A higher open area generally helps reduce pressure loss, but the final result also depends on:

• cell shape;
• foil thickness;
• cell alignment;
• frame blockage;
• inlet conditions; and
• surface finish.

The surrounding frame should be included when evaluating the effective flow area of a complete assembly.

5. Material

Material selection depends on the operating environment.

For water tunnels or corrosive process streams, material compatibility may be more important than minimum weight.

6. Joining Method

Metal honeycomb may be produced by:

• adhesive bonding;
• resistance welding;
• laser welding;
• brazing; or
• soldering.

The joining process influences:

• structural integrity;
• temperature capability;
• electrical continuity;
• corrosion behaviour;
• cleanliness; and
• cost.

A low-temperature ventilation panel and a high-temperature turbine test rig should not be specified in the same way.

Pressure Drop Must Be Considered

A flow straightener improves direction by forcing the fluid through many passages. That process consumes pressure.

Pressure loss depends on:

• cell size;
• core depth;
• foil thickness;
• open area;
• velocity;
• surface roughness;
• inlet turbulence; and
• frame design.

This creates a design trade-off:

Stronger flow conditioning generally requires more interaction between the fluid and the cellular structure, but more interaction can also increase pressure loss.

For fan systems, the straightener should therefore be evaluated together with the fan operating point.

For wind tunnels, the straightener should be evaluated as part of the total circuit energy ratio and settling-chamber design.

Where Should the Honeycomb Be Installed?

The best position depends on the complete system.

In general, a flow straightener should receive flow that is distributed across its frontal area. Installing it immediately after a severe bend, directly against a fan or in a partially blocked duct may create:

• uneven loading;
• local separation;
• non-uniform pressure loss;
• persistent high-speed regions; and
• vibration.

A plenum, diffuser, screen or appropriate straight length may be required upstream or downstream.

For precision applications, computational fluid dynamics or physical testing may be needed.

Common Applications

Honeycomb flow straighteners are used in:

• low-speed wind tunnels;
• aeroacoustic wind tunnels;
• water tunnels;
• fan test systems;
• HVAC test rigs;
• flow meters;
• cooling-air systems;
• industrial process ducts;
• exhaust-flow measurement; and
• laboratory research equipment.

Different applications may use similar geometry, but their acceptance criteria can be very different.

Common Specification Mistakes

Mistake 1: Requesting Only the Outside Dimensions

A drawing showing only length and width is not enough.

The supplier also needs the cell size, depth, material and foil thickness.

Mistake 2: Assuming Smaller Cells Are Always Better

Smaller cells may improve control but can increase pressure drop and blockage.

Mistake 3: Using “Laminar Flow” as the Only Requirement

The required result should be defined more precisely, for example:

• maximum swirl angle;
• velocity uniformity;
• turbulence intensity;
• pressure-drop limit; or
• measurement repeatability.

Mistake 4: Ignoring the Frame

A poorly designed frame can introduce more blockage and non-uniformity than the honeycomb core.

Mistake 5: Copying a Wind-Tunnel Specification into a Fan Duct

The best design depends on the inlet conditions and system objective.

Manufacturing Perspective

HengShi supplies custom metallic honeycomb flow straighteners and related stainless-steel honeycomb cores for wind tunnels, fans, test rigs and industrial ducts.

Typical configurable parameters include:

• cell sizes from approximately 0.8 to 30 mm;
• foil thicknesses from approximately 0.05 to 0.20 mm;
• circular, rectangular and custom profiles;
• resistance-welded and vacuum-brazed construction; and
• framed and unframed assemblies.

These ranges describe manufacturing possibilities rather than a recommendation for every system.

The final selection should be based on flow conditions and installation requirements.

Information Needed for a Quotation

Please provide:

1. flow medium;
2. duct or tunnel dimensions;
3. maximum velocity;
4. operating pressure;
5. operating temperature;
6. required cell size;
7. required core depth;
8. foil thickness;
9. material;
10. maximum acceptable pressure drop;
11. required uniformity or swirl criterion;
12. frame and mounting details; and
13. annual quantity.

When cell size and depth are not yet defined, provide the operating information so that the design can be discussed.

Conclusion

A honeycomb flow straightener works by dividing a large, disordered flow into many controlled passages.

Its primary purpose is not to promise “perfect laminar flow.” Its purpose is to:

• reduce swirl;
• suppress transverse velocity;
• improve directional alignment; and
• support more uniform downstream conditions.

The best results usually come from designing the honeycomb as part of a complete system that may also include screens, plenums and contractions.

Correct performance depends on balancing cell size, depth, foil thickness, open area, strength and pressure loss.

Related Products

• Stainless Steel Honeycomb Flow Regulator
• Metal Honeycomb Core
• Stainless Steel Honeycomb Core
• Contact HengShi Engineering

References

1. J. Scheiman, Comparison of Experimental and Theoretical Turbulence Reduction Characteristics for Screens, Honeycomb, and Honeycomb-Screen Combinations, NASA Technical Paper 1958, 1981.
2. G. Johl, M. Passmore and P. Render, “Design Methodology and Performance of an Indraft Wind Tunnel”, The Aeronautical Journal, Vol. 108, 2004, pp. 465–473.
3. J. Groth and A. Johansson, “Turbulence Reduction by Screens,” Journal of Fluid Mechanics, Vol. 197, 1988, pp. 139–155.

Request an Engineering Evaluation

HengShi manufactures customized metallic flow straighteners for wind tunnels, fans, test rigs and industrial ducts.

Please contact HengShi Engineering and provide your drawing, airflow conditions, material, cell size, core depth, pressure-drop limit and quantity for review.