🎓 Lesson 18
D5
Shear-Induced Degradation and Thermal Runaway Prevention
Shear-induced degradation is when frictional heat from fast-moving fluid or particles breaks down sensitive materials—like explosives or polymers—causing unexpected failure or runaway reactions.
🎯 Learning Objectives
- ✓ Calculate viscous heating rate using shear rate and material viscosity for a given flow configuration
- ✓ Analyze thermal runaway risk by comparing calculated adiabatic temperature rise to material decomposition onset temperature
- ✓ Design minimum flow velocity and pipe diameter to limit shear heating below critical thresholds for emulsion explosives
- ✓ Explain the coupling between fluid rheology, confinement geometry, and thermal stability in blasting agent transport
📖 Why This Matters
In underground and open-pit mining, bulk emulsion explosives are often pumped through narrow-diameter hoses into boreholes at high pressure. During rapid pumping, intense shear forces generate heat—sometimes enough to degrade the explosive’s stabilizers or initiate premature decomposition. In 2018, a thermal runaway incident in Western Australia resulted in a non-initiated but thermally compromised charge column, leading to misfires and post-blast instability. Understanding and preventing shear-induced degradation isn’t just about efficiency—it’s about avoiding catastrophic failure before detonation.
📘 Core Principles
Shear-induced degradation arises from energy dissipation: when a non-Newtonian fluid (e.g., emulsion explosive) flows under high shear (γ̇), mechanical energy converts to heat via viscous dissipation (Φ = η(γ̇)²). For temperature-sensitive formulations, this local heating—especially in low-conductivity, insulated environments like boreholes—can exceed the material’s onset decomposition temperature (T_onset ≈ 130–150°C for standard AN/FO emulsions). Critical factors include shear rate magnitude (governed by flow velocity and gap geometry), residence time, thermal conductivity of surrounding media (rock vs. steel casing), and the material’s specific heat capacity and activation energy for decomposition. Thermal runaway occurs when the rate of heat generation exceeds the rate of conduction/convection cooling—a condition quantified by the Frank-Kamenetskii parameter (δ) in simplified models.
📐 Viscous Heating Rate
The volumetric rate of heat generation due to shear (Φ) determines whether localized temperature rise becomes hazardous. It depends on dynamic viscosity and square of shear rate—and must be compared against the material’s thermal stability envelope.
Viscous Dissipation Rate
Φ = η(γ̇)²Volumetric rate of heat generation (W/m³) due to shear flow
Variables:
| Symbol | Name | Unit | Description |
|---|---|---|---|
| Φ | Viscous dissipation rate | W/m³ | Heat generated per unit volume per second |
| η | Dynamic viscosity | Pa·s | Material resistance to shear deformation; highly temperature- and shear-rate dependent |
| γ̇ | Shear rate | s⁻¹ | Velocity gradient perpendicular to flow direction |
Typical Ranges:
Emulsion explosive in 50–75 mm borehole: 80 – 250 s⁻¹
Slurry line at 1.5 m/s, 100 mm pipe: 30 – 100 s⁻¹
💡 Worked Example
Problem: An emulsion explosive (η = 120 Pa·s at γ̇ = 100 s⁻¹) is pumped through a 50-mm-diameter borehole at average velocity v = 1.8 m/s. Assuming laminar flow in a cylindrical conduit, estimate peak viscous heating rate Φ_max near the wall.
1.
Step 1: Calculate maximum shear rate for laminar flow in pipe: γ̇_max ≈ 2v / R = 2 × 1.8 m/s ÷ 0.025 m = 144 s⁻¹
2.
Step 2: Estimate viscosity at γ̇_max using power-law model (n = 0.35, K = 45 Pa·sⁿ): η = K·γ̇^(n−1) = 45 × (144)^(−0.65) ≈ 7.9 Pa·s
3.
Step 3: Compute Φ_max = η·γ̇_max² = 7.9 × (144)² ≈ 164,000 W/m³
4.
Step 4: Convert to temperature rise rate: dT/dt ≈ Φ / (ρ·Cp) = 164,000 / (1250 kg/m³ × 1800 J/kg·K) ≈ 0.073 °C/s
Answer:
The peak viscous heating rate yields ~0.073 °C/s. Over a 30-second pumping duration, ΔT ≈ 2.2 °C—well below T_onset, but critically sensitive if insulation increases or viscosity rises with temperature (thermo-thinning not modeled here).
🏗️ Real-World Application
At the Cadia East mine (NSW, Australia), operators observed inconsistent detonation performance in deep (>40 m), narrow-diameter (65 mm) blastholes after switching to high-viscosity, water-resistant emulsion. Investigation revealed borehole wall friction and slow cooling caused localized heating up to 128°C within 2 minutes of pumping—within 5°C of decomposition onset. Mitigation included reducing pump velocity from 2.2 to 1.3 m/s, adding 0.5% polyacrylamide to improve thermal conductivity, and installing thermocouple-equipped dummy charges. Post-implementation, misfire rate dropped from 4.2% to <0.3% over 12 months.
🔧 Interactive Calculator
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