Introduction
During operation, it is practically impossible to maintain the required heat transfer fluid temperature in the passenger car boiler. This is due to the shutdown of high-voltage electric heating tubes of the heating system. The reasons for the shutdown are different during moving and standing with subsequent heating, resulting in transient heating modes that cause temperature fluctuations in passenger cars. This process is compounded by the fact that during moving and standing of a passenger car the required heating capacity of the heating system is different, the higher the speed of the car, the more the required heating capacity of the heating system increases, this is confirmed by the results of the study [4], Fig. 1.
Transitional modes are compounded by the large heat inertia of the heating system. The installed high-voltage heating tubes also have heat inertia, which also affects the inertia of the heating system. The installed sensors for monitoring the temperature in the car have no correction for the inertia of the heating system.
Fig. 1. Required heating capacity of the passenger car heating system in relation to traffic at various outdoor temperatures
Since 2004,
all types of cars are equipped with air-conditioning units [2]. The
water heating system
with smooth regulation of the boiler’s heat output in the range of
loads from «0» to 48.0 kW is proposed. For this purpose, the
boiler is equipped with three groups of electric heating: I group –
24.0 kW, 3 kV, II group – 12.0 kW, 3 kV, III group – 12.0 kW,
380 V. The third group of the boiler is connected to the air
conditioner converter circuit through the appropriate switching
devices. The converter provides for smooth regulation of the output
power from «0» to Pnom
(12 ÷ 17 kW). Therefore, the III group of the boiler will have
smooth power regulation from «0» to
.
As a result, the entire range of heat output of the boiler will be
with smooth regulation.
The disadvantage of the proposed system is that the system can work qualitatively when heating the boiler only with electricity.
The work [2] considers the water heating system with smooth regulation of heating output of heating pipes at boiler combined heating. The system is designed for smooth regulation and stabilization of the heat transfer fluid temperature at the inlet to the heating devices, respectively, at transient and steady-state modes, stabilizing the air temperatures inside the car during cycling of high-voltage heating groups of the boiler, and also when the boiler is operated with solid fuel.
Fig. 2. Layout of
water heating with separation of flows: 1
– boiler; 2
– divisor; 3
– circulating pump; 4
– heating pipes; 5
– air line; 6
– feeding pipe; 7
– air heater; 8
– mixer;
9
– bypass pipe; 10
– expander; 11
– overflow pipe;
12
– drain plug
The principle of operation of the system, whose layout is shown in Fig. 2, consists in the separation of the heat transfer fluid flow at the inlet to the boiler into two adjustable parts by the divider 2 with an electric drive. One part of the flow returns to the boiler 1, and the second one through the bypass pipe 9, passing the boiler, goes to the mixer 8. The movement of the heat transfer fluid through the system is carried out by a circulation pump 3. Depending on the required heating output of the system, a heat transfer fluid with the required temperature is fed into the heating devices through the mixer 8. In this case, there is no need for strict maintenance of any value of the heat transfer fluid temperature at the outlet from the boiler.
This system can only work when the circulation pump is on, which reduces its reliability.
The work [8]
proposes to use the heating system
as a cold and heat accumulator; the selection
of accumulated, cold or heat is assumed only through the water
heater, without additional conversion of the heating system. Using a
heating system as a cold accumulator is possible, but as a heat
accumulator without additional conversion is impossible, since the
heating system will work in the usual heating mode. The publication
of specialists
[1] is devoted to the interaction of heating and ventilation
systems; the work proposes to automate the operation of the water
heater to maintain
the set temperature, the outer air heated in the heater and fed by
the ventilation system.
Improvement of
the operation of refrigerating equipment of passenger cars in
transitional modes was considered in [11].
The study
[14] is aimed at
reducing the energy costs of modern light rail system Luas operating
in Dublin, Ireland, through the optimization of the heating and
ventilation system. The work [12]
studied the Swiss railroad vehicles of Swiss Rhaetian Railway (RhB).
The research is aimed at improving
the energy efficiency of life-support systems for railway vehicles,
by reducing the consumption
of outside air by way of controlling
content in the air. The publications
[5, 9,
13]
are devoted to the improvement of air conditioning systems of modern
passenger cars. Problems of increasing the comfort of passenger
transportation are also examined in the scientific works of other
researchers [7, 10],
but they consider other aspects of comfort, which are associated
more with the improvement of the dynamic characteristics of
passenger
rolling stock.
Proceeding from the foregoing, it follows that along with comparatively large volumes of research on improving the design of passenger car heating systems [1, 2, 4, 8, 11, 12, 14], the tasks of ensuring the maintenance of a stable temperature condition in passenger premises are practically not considered and require more effective solutions. Therefore, the direction of research related to improving the efficiency of the operation of heating units for passenger cars is an urgent scientific and applied task.
Purpose
The purpose
of this study is to develop technical
solutions to improve the design of the water heating system, in
order to ensure a stable temperature condition in the passenger car
premises in
transitional modes of the heating
system
Methodology
To carry out the research it was necessary to solve the following tasks:
– Analyse the shared participation of convective and radiant heat exchange on the surface of heating pipes;
– Evaluate the heat transfer efficiency of heating pipes, when installing the regulating casing.
On the
heating pipe surface there is the convective
heat exchange with the internal air and the radiant heat exchange
with the walls and internal
equipment of the car. The intensity of such heat exchange is
estimated by the heat transfer coefficient,
,
[6]:
(1)
where
– radiant heat-transfer coefficient,
;
– convection coefficient,
.
For different
bodies the radiation factor is different
[3]. Its value is determined by the nature of the body, its surface
state and temperature. This law establishes the dependence of the
density
of the body integral radiation flux on its temperature.
For an absolutely blackbody:
, (2)
where
– Stefan-Boltzmann constant
.
In technical calculations, this law is applied in the more convenient form:
, (3)
where
– blackbody coefficient.
.
Consequently, the radiation energy of the absolutely black body is proportional to the fourth power of the absolute temperature.
This law can be applied to real bodies. In this case, it takes the form:
. (4)
Here с is the radiation factor of various bodies. It varies from 0 to 5.67.
Comparing the
flux density of the body’s self-radiation
with the radiation flux density of an absolutely
black body at the same temperature, we obtain another characteristic
of the body, which is called the emissivity factor
.
. (5)
The value of ε varies from 0 to 1, which is convenient.
In this case we have for a real body:
. (6)
The flux of
radiant heat exchange between two grey surfaces in an enclosed
space, when one of the surfaces with the area
flows around the other with the area
,
is calculated by
the formula:
, (7)
Where the expression:
, (8)
is called the reduced blackbody emissivity factor.
With regard to heating pipes:
– surface
of the surrounding object (i.e., surface of the walls and the
internal equipment of the car);
– surface of heating pipes.
If we consider the last formula (8) with respect to the heat transfer on the unit of the heat-sensing wall surface, the specific heat flux of the radiation will be:
. (9)
Taking into account that the specific heat flux by convection is determined by the formula:
, (10)
where
– pipe temperature, °С;
– air temperature, °С,
And the total heat flux q, by convection and radiation is as follows:
. (11)
Taking into account the expression (11), we obtain a formula for calculating the radiation heat transfer coefficient:
. (12)
In formula
(12)
is the average
temperature of bodies exchanging radiant heat.
With respect to the heat exchange between the heating pipes and air in the car, index 1 refers to the surface of the pipes, index 2 refers to the car inner equipment surrounding the pipe.
To determine
the convective heat transfer coefficient, we use the criteria for
the dependence of the Nusselt number
on the Grashof number
and the Prandtl number
:
. (13)
Having
determined,
we calculate the value of the convective heat transfer coefficient:
, (14)
where
– air thermal
conductivity coefficient,
;
d –
determining dimension, here the pipe diameter, m.
The value of
the coefficients c
and n, as
well as the product
and
in the formula
(13) is determined
by the data of the table given in [3].
Having
determined the value of heat transfer coefficient α,
we calculate the heat transfer of the heating pipes
;
. (15)
Calculations of the heat transfer fluid temperature at various points along the pipe are made using the formula:
. (16)
Let us analyse the share of convective heat transfer on the heating pipes of the heating system, the calculation results are summarized in Table 1.