From TecQuipment TE93 User Guide (2009) | Source: TecQuipment Ltd User Guide DB/bs/1109
Tags: cross-flow heat exchanger, forced convection, heat transfer, recuperative heat exchanger, TE93, TecQuipment, pitot tube, thermocouple, working section, heat exchange types
The TE93 is a bench-top cross-flow heat exchanger used to study forced convection heat transfer. A fan draws air through a duct containing aluminium rods and a heated copper rod, allowing measurement of pressures, velocities, cooling rates, and velocity profiles. The three experiments build from basic pressure/velocity calibration through to dimensionless analysis of heat transfer coefficients.
Cross-flow heat exchanger
A type of recuperative heat exchanger where the two fluid streams flow perpendicular to each other. In the TE93, air flows across heated rods arranged at right angles to the airflow.
Forced convection
Heat transfer driven by an external source of fluid motion (here, an electric fan) rather than by buoyancy differences alone.
Recuperative heat exchanger
The most common heat exchanger category, where heat passes through a separating wall between two fluids. Appears in three main forms: parallel-flow, counter-flow, and cross-flow.
Pitot tube / Pitot probe
A pressure measurement instrument that measures total (stagnation) pressure of the flow. Used with static pressure tappings to determine flow velocity.
Static pressure tapping
A small hole flush with a duct wall, measuring the local static pressure of the airflow. The TE93 has two: one upstream of the rods, one downstream.
Heated Rod
A copper rod (nominal diameter 12.4 mm) with Tufnol insulating ends and an embedded K-type thermocouple. Pre-heated in the control unit to approximately 75 °C before being placed in the working section.
Tufnol
A resin-based laminate used as the insulating end-pieces of the Heated Rod. Not a perfect insulator, which is why the effective length of the rod exceeds the exposed copper length.
Effective length (of the Heated Rod)
The copper section is 95 mm, but because the Tufnol ends conduct a small amount of heat, the rod behaves as though it were 103.4 mm (0.1034 m) long. This corrected figure is used in all heat transfer calculations.
VDAS (Versatile Data Acquisition System)
TecQuipment's optional data logging hardware and software. Connects the TE93 to a PC for real-time recording. Not essential; readings can be taken manually from the Control and Instrumentation Unit display.
Working section
The transparent portion of the duct (125 mm × 125 mm internal cross-section) where aluminium rods and the Heated Rod are inserted through drilled holes at right angles to the airflow.
Air valve
A variable valve at the fan outlet used to control duct air velocity. Graduated 0–100 %. Below 20 %, pressures are very small and readings become unreliable.
Most thermodynamic systems rely on some form of heat exchange. Engineers need to understand which type suits a given application.
The recuperative type is the most common and comes in three forms:
Parallel-flow: both fluids travel in the same direction
Counter-flow: fluids travel in opposite directions
Cross-flow: fluids travel perpendicular to each other
The TE93 demonstrates the cross-flow arrangement using air flowing over heated cylindrical rods.
The apparatus has two main units:
Control and Instrumentation Unit: houses the heater compartment (maintains the Heated Rod at ~75 °C), fan on/off controls, pressure measurement ports (two differential pressure inputs, each rated to 1200 Pa max), thermocouple sockets (T1 for inlet air, T2 for the Heated Rod), digital display, circuit breakers, and an optional VDAS connection socket.
Duct Assembly: inlet cone with effuser, working section with transparent windows, Pitot Assembly (mountable in upstream or downstream position, with digital position indicator), electric motor and fan, and exhaust duct with adjustable air valve.
The working section is a 125 mm × 125 mm square cross-section. The transparent windows have holes drilled for inserting 12.5 mm diameter aluminium rods in four columns. Five rods per column block the flow inlet when all are fitted.
Two static pressure tappings sit at the base of the working section: one upstream, one downstream of the rod positions. These connect via plastic pipework and a T-piece adaptor to the differential pressure inputs on the Control and Instrumentation Unit.
The Heated Rod is the key test specimen. Its construction:
Main body: solid copper, machined and drilled for a thermocouple
Ends: Tufnol insulating sections (23 mm top, 27 mm bottom)
Exposed copper length: 95 mm
Nominal diameter: 12.4 mm (measure accurately before each test)
Thermocouple: K-type, soldered to the copper rod internally
Mass: stamped on the end plate (copper part only, excluding Tufnol and thermocouple wire)
The effective length correction of +8.4 mm accounts for heat conduction through the imperfect Tufnol insulators. Always use L₁ = 0.1034 m and A₁ = π × d × L₁ for the effective surface area in calculations.
Working section: 125 mm × 125 mm
Duct Assembly net weight: 55 kg
Supply voltage: 230 V 50 Hz or 110 V 60 Hz
Heater compartment set point: approximately 75 °C
Secondary safety cut-out triggers at approximately 115 °C
Noise at air inlet/exhaust can reach ~86–87 dB(A); ear defenders recommended above 85 dB(A)
Air density from ideal gas law:
ρ = p_A / (R × T₁)
Where R = 287 J/kgK, p_A is barometric pressure (Pa), T₁ is ambient air temperature (K).
Effective surface area of the Heated Rod:
A₁ = π × d × L₁
Where d = measured diameter, L₁ = 0.1034 m (effective length).
⚠️ The effective length (103.4 mm) is commonly tested, not the exposed copper length (95 mm). Use L₁ in all heat transfer calculations.
⚠️ The mass stamped on the Heated Rod is the copper mass only. Do not include Tufnol or thermocouple wire mass.
⚠️ Know the three recuperative heat exchanger types (parallel-flow, counter-flow, cross-flow) and be able to explain why cross-flow is used in this apparatus.
⚠️ The working section is 125 mm × 125 mm. Five rods of 12.5 mm diameter block exactly half the inlet area. This is the basis for the velocity ratio V = 2V₁ (all rods) and V = 1.11V₁ (one rod).
Q: What are the three forms of recuperative heat exchanger?
A: Parallel-flow, counter-flow, and cross-flow.
Q: Why is the effective length of the Heated Rod longer than the exposed copper section?
A: The Tufnol end-pieces are not perfect thermal insulators and conduct a small amount of heat, making the rod behave as though it were 8.4 mm longer than the 95 mm copper section. The effective length is 103.4 mm (0.1034 m).
Q: What does the pitot probe measure, and how is it used to find velocity?
A: The pitot probe measures total (stagnation) pressure. Combined with an upstream static pressure tapping, the difference (p_t − p_u) gives the dynamic pressure, from which upstream velocity is calculated using V₁ = √(2(p_t − p_u) × 287 × T₁ / p_A).
Q: Why are readings at air valve settings below 20 % unreliable?
A: At very low flow rates, the inlet pressure is extremely small, leading to measurement uncertainty. The upstream pressure can instead be estimated from the pressure drop using the calibration gradient found in Experiment 1.
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