Transcript Slide 1

MODELING MERCURY-FREE HID LAMPS: BREAKDOWN CHARACTERISTICS
AND THERMODYNAMICS* Work supported by Universal Lighting Technologies, Inc.
*
Ayumu Sato, Nanu Brates, Koji Noro
Universal Lighting Technologies, Inc., Woburn, MA 01801 USA
Natalia Yu. Babaeva and Mark J. Kushner
University of Michigan, Ann Arbor, MI 48109 USA
CURRENT-VOLTAGE, BREAKDOWN TIMES
 High Intensity Discharge (HID) lamps are used in a variety of nontraditional applications. For automobile headlights, “instant” restart is
desired for safety considerations.
 Without salt layer
 In Hg-free HID lamps, Hg is often replaced by ZnI2 along with the use
of conventional metal halides such as NaI and ScI3.
 We discuss the properties of D4 HID lamps with results from computer
models:
 Breakdown characteristics with and without condensed salt layers,
 Mercury free D4 lamp thermodynamics database for Xe/NaI/ScI3/ZnI2
and LTE-derived densities.
 The effects of mixing, segregation and ionization of light and heavy
additives.
DESCRIPTION OF MODEL: nonPDPSIM
 Poisson’s equation, continuity equations and surface charge are simultaneously solved
using a Newton iteration technique.
       N j q j  s
j
N j
t

    j  S j

 s
   q j (   j  S j )    ( ())
t
j

 ne   
5
 
 j  E  ne  Ni i       Te , j  qe
t
2

i
 Ambipolar approximation: Continuity equations with current conservation.   ( ji )  0
 Electron energy equation
 Multiple re-strikes of the streamer during avalanche.
 For large dV/dt time of flight of seed electrons is comparable with streamer
formation time and the influence of salt layer is not very important.
 For low dV/dt time of flight is larger than the time of streamer formation - salt
layer tends to decrease the breakdown voltage and time.
Xe/NaI/ScI3/ZnI2 THERMODYNAMICS
 Transition to arc reflects change in plasma from kinetic to thermodynamic regime.
Thermodynamics of D4 mixtures are poorly understood.
 Database of Xe/NaI/ScI3/ZnI2 thermodynamic data produced to predict lamp
performance through transition from glow to arc phase.
i
 Fluid averaged values of mass density, mass momentum and thermal energy density
obtained using unsteady algorithms.
  c pT 
  v 

 


   ( v )
 NkT     v v        qi Ni Ei
  T  v c pT   Pi   v f   Ri H i   ji  E
t
t
t
i
i
i
 Individual fluid species diffuse in the bulk fluid.

 N i t  t   
   SV  S S
N i t  t   N i t      v f  Di NT 

N
T




GEOMETRY AND CONDITIONS
 Salt layer on
gravity side
 Standard D4
4 cm
lamp
2.7 mm
 Condensed salt layers on walls are present at breakdown.
Experiments show breakdown along side with salt layers.
 Salt layers (10s of μm thick) have mild electric conductivity.
PLASMA COMPOSITION vs. TEMPERATURE
 D4 lamp as used implemented in model using unstructured finite-volume mesh.
 Electron emitting edges on bottom and top electrodes.
 Voltage pulses are applied to bottom electrode with simple circuit model - ballast resistor
in series with powered electrode.
 Xe, 30 kV peak voltage, dV/dt = 150, 100, 50 V/ns, 8 atm, positive and negative
 High degree of dissociation
of ScI3, ZnI2 followed by
dissociation of heavy dimers.
 The Sc, Na, and I atoms
outstrip the molecules (3000
– 6000 K).
[e] DENSITY, NEGATIVE PULSE : dV/dt = -150 V/ns
 For T >6000 K, Sc+ (IP 6.54 eV)
and Na+ (IP=5.1 eV) dominate
over neutrals..
 Zn+ at high temperatures.
 Xe/NaI/ScI3/ZnI2 = 1/0.000316/
0.0000463/0.0000448
 No Salt Layer

[e] (3 dec)
TRANSITION TO ARC MODE
 With Salt Layer
MIN
Log
scale
MAX
No Salt Layer
 Injection of seed electrons by short puff from negative electrode.
 Electron cloud drifts towards the opposite electrode – intersects with high
field region of opposite electrode initiating avalanche.
 Symmetric discharge without salt layer.
 Conductive salt layer create regions of high electric field at edges.
 Avalanche initiated in these regions of higher E/N.
With Salt Layer
 Tracking along salt layer as a surface discharge as charging occurs.
 Multiple re-strikes to the edges of salt layer.
 Surface streamer from the opposite electrode.
Gravity
Xe/NaI/ScI3/ZnI2 =
1/0.000316/0.0000463/0.0000448
Alkali metal iodides gradually
dissociate with appearance of free
metals and free iodine.
Large special variation in the
additive vapor pressure.
Temperature gradients translate
into mole fraction variations.
Acoustic oscillations from rapid
formation of conducting channel.