1. Energy
Energy is the capacity or capability to do work and energy is used when work are done.
The unit for energy is joule - J, where
1 J = 1 Nm
which is the same unit as for work.

2. Energy Efficiency
Energy efficiency is the ratio between useful energy output and input energy, and can be expressed as
μ = Eo / Ei         (1)
where
μ = energy efficiency
Eo = useful energy output
Ei = energy input
It is common to state efficiency as a percentage by multiplying (1) with 100.

3. Example - Energy Efficiency
A lift moves a mass 10 m up with a force of 100 N. The input energy to the lift is 1500 J. The energy efficiency of the lift can be calculated as
μ = (100 N) (10 m) / (1500 J)
    = 0.67 or
    = 67 %

4. Power
Power is a measure of the rate at which work is done and can be expressed as
P = W / dt         (1)
where
P = power (W)
W = work done (J)
dt = time taken (s)

5. Since work is the product of the applied force and the distance, (1) can be modified to
P = F v         (1b)
where
F = force (N)
v = velocity (m/s)

6. Energy converted
Power is also a measure of the rate at which energy is converted from one form to another and can be expressed as
P = E / dt         (2)
where
P = power (W)
E = energy converted (J)
dt = time taken (s)

7. Example - Work done by Electric Motor
The work done by a 1 kW electric motor in 1 hour can be calculated by modifying (1) to
W = P dt
    = (1 kW) (1000 W/kW) (1 h) (3600 s/h)
    = J
    = 3600 kJ

8. Reynolds Number
The Reynolds Number is a non dimensional parameter defined by the ratio of
dynamic pressure (ρ u2) and
shearing stress (μ u / L)
and can be expressed as
Re = (ρ u2) / (μ u / L)
    = ρ u L / μ
    = u L / ν      (1)

9. where
Re = Reynolds Number (non-dimensional)
ρ = density (kg/m3, lbm/ft3  )
u = velocity (m/s, ft/s)
μ = dynamic viscosity (Ns/m2, lbm/s ft)
L = characteristic length (m, ft)
ν = kinematic viscosity (m2/s, ft2/s)

10. Reynolds Number for a Pipe or Duct
For a pipe or duct the characteristic length is the hydraulic diameter. The Reynolds Number for a duct or pipe can be expressed as
Re = ρ u dh / μ
    = u dh / ν          (2)
where
dh = hydraulic diameter (m, ft)

11. Reynolds Number for a Pipe or Duct in common Imperial Units
The Reynolds number for a pipe or duct can also be expressed in common Imperial units like
Re = u dh / ν           (2a)
where
Re = Reynolds Number (non dimensional)
u = velocity (ft/s)
dh = hydraulic diameter (in)
ν = kinematic viscosity (cSt) (1 cSt = 10-6 m2/s )

12. The Reynolds Number can be used to determine if flow is laminar, transient or turbulent. The flow is
laminar when Re < 2300
transient when 2300 < Re < 4000
turbulent when Re > 4000

13. Example - Calculating Reynolds Number
A Newtonian fluid with a dynamic or absolute viscosity of 0.38 Ns/m2 and a specific gravity of 0.91 flows through a 25 mm diameter pipe with a velocity of 2.6 m/s.
The density can be calculated using the specific gravity like
ρ = 0.91 (1000 kg/m3)
    = 910 kg/m3

14. The Reynolds Number can then be calculated using equation (1) like
Re = (910 kg/m3) (2.6 m/s) (25 mm) (10-3 m/mm) / (0.38 Ns/m2)
    = 156 (kg m / s2)/N
    = 156 ~ Laminar flow
(1 N = 1 kg m / s2)

15. Potential energy
When a body of mass is elevated against the gravitational force the potential energy can be expressed as
Ep = Fg dh
    =  m g dh         (1)

16. where
Fg = gravitational force (weight) acting on the body (N)
Ep = potential energy (J)
m = mass of body (kg)
g = gravitational acceleration (9.81 m/s2)
dh = change in elevation (m)

17. Example - Potential Energy when body is elevated
A body of 1000 kg is elevated 10 m. The change in potential energy can be calculated as
 Ep = (1000 kg) (9.81 m/s2) (10 m)
    = J
    = 98 kJ
    = kWh

18. To be continued n Thank you