Експериментальне визначення коефіцієнтів тепловіддачі при вимушеної течії повітря в трубі

Summary

This transcript describes a laboratory study to experimentally determine average heat-transfer coefficients during forced-air flow in a heated pipe. It explains convective heat-transfer theory, key dimensionless similarity criteria (Reynolds, Nusselt, Prandtl, Grashof/Archimedes, Froude), and flow regimes (laminar, transitional, turbulent). The setup uses a compressor-driven air stream, an electrically heated insulated pipe, thermocouples along seven sections, and a measuring diaphragm to find mass flow. The procedure covers steady-state measurement, data reduction (temperatures, pressure, heat flux), iterative calculation of outlet temperature, computing Nusselt numbers, applying similarity equations, plotting coefficients vs. length, and comparing theory with experiment—highlighting notable deviations in the inlet region.

Takeaways

• 🔥 The experiment focuses on determining heat transfer coefficients during forced air flow in a pipe to study convective heat transfer phenomena.
• 💨 Forced convection involves fluid movement driven by external forces like fans or pumps, distinguishing it from natural convection caused by buoyancy effects.
• 📏 Heat transfer is governed by the Newton-Richman law, which relates heat flux density to the temperature difference between a surface and the surrounding fluid.
• 📊 The heat transfer coefficient (measured in W/m²·K) represents how efficiently heat is transferred per unit area and temperature difference.
• 🔢 Several dimensionless similarity criteria—such as Reynolds, Nusselt, Prandtl, Grashof, Archimedes, and Froude numbers—are used to describe and compare heat transfer conditions.
• 🌀 The flow inside the pipe can be laminar, turbulent, or transitional depending on the Reynolds number, which is determined by flow velocity, viscosity, and pipe diameter.
• 🌡️ Heat transfer behavior changes along the pipe: it decreases in the initial thermal region and stabilizes once the flow becomes thermally developed.
• 🔍 The experiment involves measuring air temperature, wall temperature differences, pressure drops, and electrical heating parameters across multiple pipe sections.
• 🧮 Data processing includes calculating average temperature differences, air density, mass flow rate, Reynolds and Nusselt numbers, and comparing theoretical vs. experimental heat transfer coefficients.
• 📈 Results are presented in tables and graphs showing how the heat transfer coefficient depends on pipe length, allowing comparison between theory and experimental outcomes.
• ⚙️ The setup uses a heated, insulated pipe supplied with air by a compressor, with thermocouples placed along its length for temperature measurements.
• ✅ The procedure emphasizes establishing steady-state conditions, performing repeated measurements, and verifying data consistency before final analysis.

Q & A

What is the primary objective of the experiment described in the script?

The primary objective is to study heat transfer during forced convection, master convective heat transfer concepts, and acquire experimental skills to determine heat transfer coefficients in pipes.

What is the difference between forced and natural convection?

In natural convection, heat is transferred due to the natural movement of fluids caused by temperature-induced density differences. In forced convection, fluid movement is induced by external forces like pumps or fans.

What is the Newton-Rigman law, and how does it relate to heat transfer?

The Newton-Rigman law states that heat flux density is directly proportional to the temperature difference between the wall and the fluid. The proportionality constant is the heat transfer coefficient, measured in watts per square meter per kelvin.

How is the heat transfer coefficient calculated in the experiment?

The heat transfer coefficient is determined experimentally through measurements of temperature differences between the pipe wall and air at different points along the pipe. The average heat transfer coefficient is then calculated based on the experimental data and using similarity equations.

What are similarity criteria, and why are they important in the experiment?

Similarity criteria are dimensionless numbers used to scale physical processes and phenomena. They help compare experimental results with theoretical predictions, providing insights into the relationship between various fluid dynamics and heat transfer parameters.

What is the Reynolds number, and how does it affect the flow regime in the pipe?

The Reynolds number is a dimensionless number that indicates the flow regime within a pipe. If the Reynolds number is less than 2000, the flow is laminar; if greater than 10,000, the flow is turbulent; and if between 2000 and 10,000, the flow is transitional.

How does the experiment account for changes in temperature and fluid velocity along the pipe?

The experiment measures the temperature and velocity at various sections along the pipe and calculates the corresponding heat transfer coefficients. The temperature difference between the wall and the fluid is also recorded to understand how these factors affect heat transfer along the pipe.

What is the significance of the Prandtl number in this experiment?

The Prandtl number, which relates the fluid's momentum diffusivity to its thermal diffusivity, is used to characterize heat transfer in the fluid. It is important for determining the length of the initial thermal section and calculating the local heat transfer coefficient in the experiment.

What are the steps involved in determining the heat flux in the experiment?

The heat flux is determined by calculating the temperature differences between the air and the wall at various points, then using the average temperature values in the formula to find the heat flux density. The process involves iterative calculations until the outlet temperature stabilizes.

How is the experimental setup designed to measure heat transfer?

The experimental setup includes a pipe through which air flows, with thermocouples to measure air temperature and temperature difference across various sections. The pipe is heated electrically, and the air flow rate is monitored using a pressure drop across a measuring washer. The setup ensures the controlled measurement of heat transfer coefficients.