Transcript DETERMINATION OF HEAT TRANSFER COEFFICIENTs UNDER

DETERMINATION
OF HEAT TRANSFER COEFFICIENTS UNDER
CLIMATIC CONDITIONS OF ISTANBUL
Mustafa Kemal Kaymak and Ahmet Duran Şahin
İstanbul Technical University, Aerospace and Aeronautics Faculty,
Meteorology Department, Energy Group, 80626 Maslak/İstanbul
[email protected], [email protected]
Atmospheric Conditions and Natural
Gas Consumption
• Main natural gas consumption amount occurs in
winter.
• Consuming natural gas or other fossil/ renewable
sources for heating means to fight against
atmospheric conditions.
• Generally atmospheric temperature is taken the
main variable to effect natural gas consumption
• But some of others are forgotten for example
wind
Main Aim
• In this research cooling effect of wind speed to buildings is considered under
climatic conditions of İstanbul in detail. Firstly, external surface heat transfer
coefficient is calculated depends on wind velocity changes.
• As known that depend on seasonal variation wind speed effects also changes. In
other words in winter time wind speed values cause to high level heat losses and so
additional heating required.
• In contrast to this, in summer time wind speed values cause to additional cooling
and could be used as supporter for air conditioning of buildings. Depend on these
conditions cooling and heating estimations should be considered for energy
planning.
Wind chill and Its Importance
• The Wind Chill Index is based on an equation first proposed in 1939 by
Paul Siple, a famous geographer, polar explorer, and an authority on the
Antarctic. In the 1940s, he and fellow Antarctic explorer Charles F.
Passel conducted experiments on the amount of time it took for water
to freeze in a plastic cylinder while exposed to the elements. They
discovered that the time it took for the water to freeze depended on
the initial temperature of the water, the outside air temperature, and
the speed of the wind.
• Moving air carries heat away from the body more effectively than air
that is not moving. If there is no wind, the heat radiating from a
person's body will stay near the body and warm the air around it.
Therefore, the wind chill is simply a means of describing the effect of
the movement of air on the heat loss of a body.
• The wind chill factor or equivalent temperature uses a neutral skin
temperature of 33 °C as a baseline value.
Heat transfer coefficient and Wind chill index
related with human body
• Knowing the latent heat of fusion of water, the surface area of the
cylinder, and the air temperature, they are able to calculate heat transfer
coefficients, which is called as wind chill factors, hwc. This factor is related
to the wind speed, V (m/s), by an emprical equation
hwc  10.45  10 V  V
• The wind chill index (WCI), which is an estimate of heat transfer, is
obtained by multiplying the wind chill factor (hwc) by the difference
between an assumed skin temperature (33°C) and air temperature (Tair)
WCI  hwc (33 C  Tair )
Heat Losses and Structures
• As known that buildings could be considered as complex structures
for heat transfer that continuously varies with weather conditions,
flow field and surface geometries.
• It is well known that climatic comfort sensation in a room is not
only dependent on the indoor air temperature, but also on the
inner and outer surface temperatures.
• Under passive heating or cooling conditions, indoor air and surface
temperatures change with the rate of heat flow through the
building envelope is the function of the thermo physical and solar
radiation properties of the opaque and transparent parts.
• Airflow around buildings affects weather and pollution
protection at inlets, and the ability to control
environmental factors of temperature, humidity, air
motion, and contaminants.
• Wind causes surface pressures that vary, around buildings,
changing intake and exhaust system flow rates, natural
ventilation, infiltration and exfiltration, and interior
pressure. The mean flow patterns and turbulence of wind
passing over a building can cause a recirculation of
exhaust gases to air intakes.
• If a building is oriented perpendicular to the wind, it can
be considered as consisting of several independent
rectangular blocks.
• As known that topographical conditions are very
important for calculation of wind profile at
different heights. Roughness length represents
sea, shore, land/complex topography and urban
areas effects to the wind profile. For urban area
roughness length is suggested to consider as 1.5 m
or over this value.
• The mean speed of wind approaching a building
nonlinearly increases with height above the
ground. Both the upwind velocity profile shape
and its turbulence level strongly influence flow
patterns and surface pressures.
DATA AND STUDY AREA
• In order to evaluate and assess heat losses depend on wind speed
values, a database is considered of hourly average wind speed and
direction measurements taken between 1996 to 2006 in the northern
part of Istanbul, which is located between 40.97E longitude and
29.08N latitude.
• This area comes under the influence of the mild Mediterranean
climate during the summer, and consequently experiences dry and hot
spells for four to five months, with comparatively little rainfall.
• During the wintertime, this region comes under the influence of highpressure systems from Siberia and the Balkan Peninsula and lowpressure systems from Iceland. Hence, mainly northeasterly or
southerly winds influence the study area, which also has high rainfall
amounts in addition to snow every year in winter. Air masses
originating over the Black Sea also reach the study area.
Kumköy Meteoroloji İstasyonun Rüzgar Gülü
N
NNW 12000
NNE
10000
NW
NE
8000
6000
WNW
ENE
4000
2000
W
E
0
WSW
ESE
SW
SE
SSW
SSE
S
Figure Wind rose for considered region
APPLICATION
• As known that wind profile in other words wind speed changes with
height depend on topographical conditions. These conditions are
summarized with roughness length that represents surface friction and
topographical complexity.
• Roughness length changes between 0.00001 m to 2.5 m from sea level
to very high complex urban areas respectively.
• In this paper, depends on complex urban conditions roughness length
(z0) is considered as 1.5 m for wind profile estimation. It should be
remembered that wind speed values (from reference height to desired
hub height) show higher changes at complex conditions than sea level.
• Wind speed profile at desired height could be estimated by using
logarithmic approximation as
ln( Z 2 / z 0 )
V2  V1
ln( Z1 / z 0 )
In here, V1 represents measured wind speed values
at 10 m that is reference level for this paper and V2
represents wind speed at desired building height.
• In addition to this, as mentioned before roughness length
changes from sea level to complex urban topographical
conditions.
• In this paper wind speed profile is evaluated in complex
urban conditions at different heights which are considered
10m, 30m, 60m, 90m, 120m, 150m, 180m and 210m.
• It is estimated that in the afternoon wind speed values
reach to the maximum value and considering these data
there is a suddenly jump between 10 m and 30 m heights
after this level variation is getting stable.
12.0
Wind Speed (m/s)
10.0
10 m
8.0
30 m
60 m
6.0
90 m
120 m
4.0
150 m
180 m
210 m
2.0
Figure Wind profile changes
during day for different
heights in Sarıyer-İstanbul
0.0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Hour
•
In order to satisfy the minimisation of supplementary mechanical energy demand, the
optimum value of the overall heat transfer coefficient for opaque components should
be determined in terms of the climatic conditions of the predominant period for the
region.
•
To select the predominant period, it is necessary to compare the under heated and
overheated periods, by taking their duration and severe climatic conditions into
account.
• Calculation of external surface heat coefficient,
hw at different heights of buildings depends on
wind velocity is the main approach of this study.
• The American Society of Heating, Refrigeration
and Air-Conditioning Engineers (ASHRAE), in
their most recent revision of the ASHRAE
Handbook, recommend a standard value for the
external convection coefficient, hw , of 29 Wm2K-1 at a wind speed of 6.7 ms-1.
for the winward side
for leeward as
hw = 7.4 + 4.0 V
hw = 4.2 + 3.5 V
60.0
hw (W/m2K)
• In this paper, hw values at
50.0
different heights are
estimated based on above
40.0
equations. It is shown in the
hw_10 m
hw_30 m
30.0
next Figure changing of hw
hw_90 m
and wind velocity with
hw_180 m
20.0
height.
hw_210 m
10.0
• It is easily seen that there is
an increase values of hw
0.0
with height of building. For
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Hour
windward side of building
above 30 m during the day Figure Heat loss coefficient variation during day for
all convection coefficients different heights in Sarıyer-İstanbul on windward side.
are higher than standard
ASHARE value.
• In other words, over 30 m height buildings
standard ASHARE value that is 29 W/m2K, should
be rearranged in İstanbul conditions. Additionally,
if height of building is increased differences
between levels will be increased.
For leeward side of the building approximately 25% less
than windward heat losses occur. Atmospheric boundary
layer conditions also effect low level urban conditions on
the leeward side.
45.0
40.0
35.0
hw (W/m2K)
30.0
hw_10 m
25.0
hw_30 m
20.0
hw_90 m
hw_180 m
15.0
hw_210 m
10.0
5.0
0.0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Hour
Figure . Heat loss coefficient
variation during day for
different heights in Sarıyerİstanbul at leeward side.
Heat transfer coefficient and Wind chill index
related with structures
• External surface heat coefficient, hw let us to know wind chill factors for buildings.
This factor is related to the wind speed, V (m/s), by an emprical equation for
windward of building
• hwi  7.4  4.0Vi and for leeward of building hw  4.2  3.5Vi
i
• The wind chill index for building (WCB), which is an estimate of heat transfer,
could be obtained by multiplying the wind chill factor for buildings (hw) by the
difference between an assumed building envelope temperature (14.98°C) and air
temperature (Tair)
WCBi  hwi (14.98 C  Tairi ) Summation of WCB gives heating or cooling degree of the
building.
n
n
i 1
i 1
  WCBi   hwi (14.98 C  Tairi )
  0.0
if 
  0.0
less heating or
cooling
heating
require
require
In short, in İstanbul conditions buildings losses
much heat energy than gains
Total WCB Values for different heights and hours
600000
Heating or Cooling, W/m2K
500000
400000
300000
200000
100000
0
-100000
-200000
-300000
02:00
08:00
14:00
20:00
σ, 30 m
190696.33
129451.42
-100867.43
106018.65
σ, 120 m
450148.41
311664.78
-181503.33
152676.61
σ, 210 m
504485.35
349586
-186645.16
186883.74
CONCLUSIONS
• In this paper external heat transfer coefficient of the building
envelope is considered under climatic conditions of İstanbul.
• It is estimated that over 30 m height ASHARE standard heat
convection coefficient for building envelope should be
rearranged in İstanbul.
• Additionally, complex topographical conditions effects are
decreased above 120 m in the boundary layer of İstanbul.
• For the perspective of heating and cooling due to the wind
speed values maximum heat losses occur after midnight
hours.
• A new approache related with wind chill index suggested for
building heating or cooling requires.
Thank You